It's no coincidence that universe 13 has Vegeta profiting the most. It is meant to show how much the main Vegeta has grown as a character. U13 Vegeta has everything Vegeta originally wanted before he reformed. Vegeta defeated Freeza by being the first one to become a Super Saiyan. Kakarot still became really strong just like in the main universe except he never surpassed Vegeta and remains second best to Vegeta. He even becomes the ruler of his universe, gets to roam the universe and fighting like he talked about doing when he first introduced in the manga. When the two Vegetas meet the main one acts unimpressed not out of jealousy, but because he no longer cares about the things he once did in the past. The novelization of the comic actually shows U13 Vegeta feels envious of the main one for having a family, which he doesn't have. Vegeta is completely satisfied with his life on Earth now.
Universe 9's strongest warriors being humans is actually a nice case of Fridge Brilliance. The first thought of a universe where Goku never came to Earth would mean the human race would be wiped out by one of the galactic threats, universe 9 actually offers an alternative of the human warriors winning on their own. This isn't such an impossibility when you think about it for a few reasons.
Even without Goku coming to Earth most of his friends would still be on the same path without him. Bulma would still look for the Dragon Balls so still encounter Yamcha. Krillin would still journey to train under Roshi. Tien was of course already training under the Crane Hermit. They would still be in the same positions they were in canon and still be the ones to face Earth based threats before the space threats came.
Its implied that Krillin trained under King Kai instead of Goku to fight the Saiyans. While some would think Krillin would lose to Vegeta and Nappa even with King Kai's training it is important to remember that the human characters were pretty close to Goku in power before he got trained by King Kai. It was shown that when they receive the same training Goku got that they improved at same drastic rates (at least before the Namek trip, when zenkai boosts pushed Goku too far ahead for them to catch up). The idea of Krillin reaching a similar level Goku reached when he trained under King Kai isn't so far fetched.
The Saiyans and Piccolo being absent might have forced the humans to train harder than they did in the main universe. This a subtle development hinted at in the series. Tien turned down training in the Time Chamber, because he thought it was a waste of time since he couldn't hope to catch up to the Saiyans or Piccolo. Yamcha and Krillin gave up fighting knowing they could never catch up. However, in a universe where they are Earth's last defense they would been forced to train even harder if they wanted to save the Earth. Krillin being elderly now he is still a active fighter who can easily defeat one of Cooler's elite warriors. Tien who is even older is still active as well and implied to be a rival to Yamcha, who has become a cyborg.
While Krillin and the others are shown to not be anywhere near the level as the U16 or U18 Saiyans they attributed most of their victories against the same set of enemies that they faced to teamwork and tactics. This is a nod to how the human characters have shown to use more tactics, have more creative techniques, and be less prone to Blood Knight tendencies than the Saiyans. They probably wouldn't be afraid to take cheap shots to win or weaken their enemies to win unlike, say, Vegeta or Goku.
The obvious one of Yamcha being made into a cyborg would of course potentially make him extremely powerful. 17 and 18 were normal teenagers before Gero made them into cyborgs who could best Super Saiyans. A seasoned warrior like Yamcha who was already one of the strongest people on Earth being turned into a cyborg could easily put him above 17 and 18 in canon, perhaps even close to some of the Super Saiyans in power.
At first sight, Universe 2's early loss might seem weird, considering it's composed entirely of extremely powerful reality warpers (Zen Buu compares Arale's abilities with his own, and it's also stated that, had she joined the Z warriors, their fight against Cell and the androids would have been much easier). However, it must be noticed that the universe is made entirely of Rule of Funny, hence it had to lose: seeing the main characters (and most powerful fighters) defeated by joke ones wouldn't be funny at all, and even making it an hard fight can only work if it's a One-Scene Wonder (it must be noticed that the only time one of them won an actual fight is when the opponent was another character from the same universe). And that's the same reason why Arale couldn't have helped them against the androids in universe 18.
It seems odd that Freeza isn't any stronger in Universe 8, in light of "Resurrection F"...at least until when the background of how the Z-Fighters lost is revealed. In "Resurrection F", Freeza is motivated to train due to his undying vendetta against Goku, for being humiliated and bested on Namek. But in Universe 8, Freeza simply killed the Z-Fighters on Namek before Goku arrived, and had no reason to stick around afterwards; he then met Goku, missing one more Zenkai power-up he would have got against Ginyu, losing to Freeza's final form. No wonder Freeza is so outclassed in the tournament; with Krillin killing Vegeta and never being humiliated, he'd have no reason to improve! In contrast, Cold is an Adaptational Badass because he has every reason to improve; with the devastation of Freeza's capital, and later both Dr Raichi and Dabura, the two only beings who could challenge him, he'd have a reason to get better and achieve his augmented states. Not to mention, his own children aren't exactly what you would call loyal; Cooler himself expresses a desire to overthrow Cold. And as both a No-Nonsense Nemesis and far more responsible leader, he wouldn't half-ass it like Frieza.
It also helps a LOT that King Cold is NOT King Cold, but Ginyu in disguise, who actually has the drive to improve and master his power.
It might slip most people's notice, but Zen Buu has not once used anything other than his physical abilities to win his matches (not counting the Ultra Armor he swiped); he hasn't even used any ki attacks and in the case of Tien, he didn't do anything at all. Considering his tendency to show off his skills, such as when he played music for the audience, one would think he'd take every opportunity to show off all the new powers he had acquired over the past twenty years. But then take into account that XXI is set up to be the main bad guy and, more recently, we've shown that Buu and the Grand Kaioshin are working together. The most logical reason for this is that Dai Kaioshin recruited Buu, probably before the tournament even started, to help defeat XXI and indeed, Buu's next match is against him. Now there is a reason why Buu hasn't shown any of the new abilities he has: he doesn't want to give XXI any kind of advantage and so is keeping all his cards stashed away until the right moment.
In Universe 8, it might seem weird at first how Cold favors wearing a full shoulder pad armor even in his original form when Frieza and Cooler don't and remain naked. Then it becomes obvious when it is revealed that it's actually Ginyu, meaning he was probably feeling nostalgia for the same type of armor that he used to wear himself!
Babidi actually makes it a point to say that he's not going to try and control any of the Vegetas, because he remembers how Vegeta was able to shake off his control when he tried it in his universe. He even avoids Vegetto, considering someone who's half Vegeta too much of a risk. Cell being able to resist him, and even attack him, makes perfect sense since he's partially made from Vegeta's DNA.
The U16 special culminating in Bra cutting loose as a SS2 and causing a solar system-wide destruction event now gives a possible explanation for her mindset, in a Deconstruction of the series as a whole. The heroes have access to the Dragonballs, are buddies with the gods and can fix any mess, and this is true in many universes. The thing is, they all had a strong moral compass and proper parenting instigating these values in them BEFORE having access to all these nice things. Bra? Because of her power, she is only second to Gohan and Vegitto, and she always had temperament issues, not helped by the poor parenting of Vegitto. Therefore, because her flaws were not corrected and because she has always had casual access to incredible feats of power, to the gods and the Dragonballs, she can do what she wants, and any mess that could happen, she could wish away. She likely doesn't GET why it's a bad thing... since it can all be wished away. She simply never had bad consequences stick, not even when Vegitto threatened to kill her if she would turn into a menace. Since her education never properly taught her restraint or how privileged she was, her SS2 state ends up bringing out her base instincts to the forefront, where she only feels the need for raw power, which overshadows any caring about consequences, because her base thought process is "break everything first, fix afterwards what you need".
Also, for the deconstruction: Cue the logical conclusion now the Majin Rebellion is over, potentially permanent consequences are now in the middle of happening, this fact has dawned on her, and she is clueless as to how to deal with it.
The French novelization outright confirms this interpretation. U16!Bra wanted to cut loose, destroy planets to let off some steam, but people wouldn't let her; She didn't undestand why: they can come back to life thanks to the Dragonballs! U18!Bra rips her a new one over the fact for her death is just a game, outright daring her to kill both her and U18!Goten, and to imagine what it would feel to kill her own mother. The Armor-Piercing Question is what has her realize she is Drunk on the Dark Side.
U17!Cell vs U18!Vegeta has Cell bring the possibility of moving his core around, meaning Vegeta has to pinpoint it from the way Cell guards against damage. This addresses the initial Plot Hole in canon where Goku had blasted his head via Instant Transmission Kamehameha, yet Cell survived.
In Universe 7, Gast Carcolh killed Freeza, which is all well and good...except Goku won't be able to achieve SSJ in this timeline. Even if Goku doesn't die from the heart virus, the Z-Fighters will have no way to stop the androids since Gast is most likely off-world. Trunks isn't coming to save them since, if Gast didn't end up killing him outright, Vegeta isn't going to sleep with Bulma in this universe. Not to mention that Cell, which will be quite different since he won't have Frieza's and Cold's DNA, is going to have nothing stopping him from absorbing the androids. While it's stated that Gast stopped what should be his version of Cell (and later Majin Buu), in all likelihood humanity is extinct.
Cell would also be different from his normal incarnation, since he would only have Namekian cells from Piccolo, but none from Freeza or Cold..
Universe 10 is pretty much the same as Universe 1, however its strongest fighters are total jokes... but the Supreme Kais, who should exist in that universe, aren't part of the competition. We know from the Gotenks vs Gotenks fight that even nigh-identical counterparts are fair game, so the U10 Supreme Kais must be dead. Considering they preemptively struck down any being who could be a threat, what the Hell wiped them out?!
It's possible that they just turned down the invitation.
The novelization confirms that the divergence occured before Broly got killed.
Universe 13 finally reveals Kakarot managed to wish for immortality. It is known he has gone crazy from being alone with the destructive goal of killing all humans, but now this might imply that he has spent years starving, suffering yet incapable of dying; all manner of Who Wants to Live Forever? horror. Maybe this played into his insanity?
It also makes it clear why U13!Vegeta is downright gleeful at butchering the ghost of U3!Kakarot: now we know he has to put up with HIS Kakarot because he can't die, and it makes him MAD. This was his first opportunity for release in an eternity.
Raditz's line about being unable to kill Kakarot brings some snarky and doubtful comments from U9!Tenshinhan about Raditz having a heart, the mere notion making the Saiyan scoff. Turns out he is not wrong as Raditz does have a modicum of brotherly feelings, but there was a more literal meaning all along...
The implication is also that Kakarot has used the Dragonballs at least twice, since he knew they turn to stone for one year after being used. We know one wish made him immortal, but what were the others and how many did he make?
We know that U19 is fighting to save their universe by preventing their Grey Goo planet from spreading carbonite throughout the whole universe. However...
This goes in this territory when we realize the carbonite was used in U3 to get rid of Broly, meaning the stuff exists in a LOT of universes, and is likely an unaddressed threat EVERYWHERE else.
when Frieza and Cooler don't and remain naked. Then it becomes obvious when it is revealed that it's actually Ginyu, meaning he was probably feeling nostalgia for the same type of armor that he used to wear himself!