Some tracks seem to channel Star Wars, which isn't a bad thing.
Demonic Spiders: The torpedo seaplanes in the Core Contingency expansion, which are torpedo bombers on steroids. They have the flight capabilities of a Brawler mixed with the ability to drop multiple torpedoes and fire rockets. If you're playing on the Hydross maps, land units cannot be built, so you're reliant entirely on ships and floating defense platforms to take them down. The only thing that can reliably shoot down torpedo seaplanes are missiles, and only three units possess that ability: scout boats, which are hilariously fragile and don't do much damage; floating missile towers, which are also fragile and can't evade like the scouts can; and missile frigates, which are sluggish, pricey, fragile (noticing a trend?), and fire slowly. Oh, and since they aren't sea units, torpedo seaplanes don't show up on sonar, making it nigh-impossible to know they're coming before you see a dozen torpedoes heading straight for your underwater power plant or submerged Commander — and torpedoes hurt. The only real solutions are either to riddle sea areas with floating missile towers or crank out your own swarm and dogfight them to death. And just for one last added annoyance, seaplanes can easily turn into that last invisible patrolling unit you have to scour the map to kill. The Core at least gets a dedicated anti-air ship (which is also fragile, but at least decently fast), which helps out once you get a few of them out, but even then they're a major threat.
First-tier defense turrets during the first minutes of a game. Later on they devolve into Goddamned Bats that can be wiped out fairly effortlessly, but in the early stages a few strategically placed pew-pew turrets can turn a moderately effective rush into a mess of critical-health units that can no longer lethally damage the enemy base.
Arm Fidos. They only pack medium firepower, but are faster and tougher than any other unit with the same offensive abilities. Their gun fires very fast projectiles too, so micromanaged enemy units can't dodge them the way they can rockets or heavy tank shells. Reliable defenses will bring down a Fido rush, but if your defenses aren't sufficiently reliable and a bunch of them slip through you will have a bad time.
Arm Spiders. They're delicate and harmless as far as damage dealt is concerned, but if you make the mistake of not prioritising them in your targeting, a few aimed hits will stop your heavy units dead in their tracks - ready for the rest of the enemy force to blow them into scrap.
Fan Nickname: Many, such as "Blue Laser of Death" for the most powerful beam weapon, and "Gator" for the Instigator light tank.
For the Arm, an early-game Peewee or Flash rush can be nigh-unstoppable, depending on how fast you can crank them out and how well-defended the opponent is.
Later in the game, the Arm Brawler can tear through almost anything, provided you have enough of them. It's tough for a fighter and the constant stream of fire will wear down nearly any defense. It won't outright win the game if the enemy has enough defense, but a decent group of them will still tear a large chunk out of an enemy base if they make it there. This is downplayed with the addition of flak cannons in Core Contingency, which will rip aircraft to shreds in seconds and stop such rushes in their tracks.
Against the AI, nukes are game over, mostly because it can't use them.
Air transports can only carry one ground unit at a time, but that includes enemy units. If an air transport carrying another unit is destroyed, the unit goes with it. Zerg Rush the enemy base with a flight of air transports, order them to pick up the Commander, and then laugh as the enemy forces continue firing, destroying their own Commander. Or use the self destruct.
Bomber rushes. A player that lets their enemy build a ludicrous enough number of bombers will find that there is no amount of fixed defenses that will reliably stop all of them. All the enemy has to do is fly a scout plane into your base and find out where your Commander is, then gather up all their bombers and target it, completely ignoring any defenses. The vast majority of the bombers will die on the way, but it only takes three or four of them to reach bombing distance and the Commander is toast. This is functionally impossible to defend against, simply because the way targeting works in the game - even area-effective flak guns won't reliably target the middle of a plane formation, rather targeting the first units they see in the front, and the killing effect is usually insufficient to grind through to the core of the enemy formation.
Goddamned Bats: Stealth field units. Not because they afford any advantage, but because you have to scour the entire map trying to find them after you've killed everything else. On larger maps, this can take upwards of thirty minutes.
Arm Zippers. They're lightly armoured but not quite as uselessly so as Flea bots, lightly armed but not quite as harmlessly so, and fast. Nobody includes them in an attack force for very good reason, but they're wonderful for infiltration and interference - if you manage to slip a few of them unnoticed into an enemy metal-gathering zone, by the time they realise their mistake and gather enough forces to wipe them out you'll have crippled their economy.
Scout planes. They literally cannot deal any damage whatsoever, but they're too fast for defenses - no matter how massed - to reliably intercept them before they irritatingly fly above your base. At some point a sufficiently Persistent Missile is likely to blow them out of the sky, but by that point the enemy already knows exactly what you're up to and you can expect trouble soon.
Good Bad Bugs: Notably, the long-range artillery can be made even longer-range by commanding it to force-fire directly in front of it, which confuses the AI into elevating the barrel and throwing shells in an arc across the map. Terrible accuracy, of course. Some mods "fix" this by increasing the artillery's max range to take advantage of this angle adjustment.
Missions where you're reliant on only sea units are notoriously annoying, since the enemy gets the stronger units right off the bat and sea units cost significantly more to build than land units, which is compounded by the fact that you only get so much metal generation, and maps with lots of open ocean tend to be very stingy with metal deposits.
Arm Mission 6: Beachhead on Thalassean. You start off on an island that is surrounded by Core Ships and must rush to build Guardian Plasma Guns or T2 Naval Units. Failing to do so results in your Commander being attacked and eventually destroyed by the Core Ships as they can just safely shell him with impunity. Even if you D-Gun the Enforcers as they approach the shore, this won't stop the missile frigates.
Arm Mission 7: Defense of Larab Harbor. Defend one massive fragile Moho Mine against oncoming waves of bombers, underwater tanks and eventually Battleships. The Bombers are especially annoying, because they only need one good sweep to blow it up and they are dedicated to that task. You start with a shipyard, but the problems with ship-building still come into play and it's a lot harder to D-Gun them like in the last mission. Further complicating things, you only have two metal deposits, counting the one under the Moho Mine. If you don't realize you need to carpet the island with Solar Collectors and Metal Makers, you're in for a bad time.
Taken up to eleven with the Hydross levels in the expansion. Not only do you have all the existing problems with naval units being ridiculously expensive, but now your resource and defense options are seriously limited while the addition of new units like the torpedo seaplanes — see Demonic Spiders for why they're a problem — make establishing a decent base ten times harder. It also takes away your Commander's ability to defend himself, since he needs to be above water to attack, forcing you to set up shop near a reef if you want to use the D-Gun.
Core Mission 4: Enough is Enough. You're tasked with destroying an Arm fortress and capturing a Galactic Gate. The fortress is on the high ground, in the center of the map, and ridiculously well fortified. If you had access to better troops, it would be easier, but the only advanced unit you can build is the Pyro, a sluggish, frail Kbot with a dubious weapon. On harder difficulties, Arm will absolutely carpet the high ground with Defenders and Sentinels and rain down hell on any attempt to assault their defenses.
Rougpelt and its goddamn meteor showers from hell. Both the Arm and the Core campaigns put you on Rougpelt for a handful of missions. Every so often (whenever the Random Number God feels bored), the game will drop meteorites somewhere on the map, damaging anything underneath them. They can't be destroyed or defended against, just weathered. Aside from the damage they'll do to your more fragile buildings and units, they might hit the enemy, which sounds good, but in practice often tends to provoke assaults or damage structures the game requires you to capture. This comes to a head in the Core mission 'Departing Rougpelt'. It's a tiny map, so there's only two places for the meteor showers to go: On you, or on the Arm fortress you're assaulting. If they land on the Arm fortress, it's likely to trigger a retaliatory strike, which could be Zippers backed up by Spiders, Bulldogs and Merls, or even a wave of Brawlers.
Core Contingecy one-ups Roguepelt with the Gelidus maps, which feature hail storms that can be even more intense and last for minutes at a time. On the plus side, assuming it doesn't hit you first, it will basically cripple the enemy if it hits them.
Core Contingency Arm Mission 6: The Beacon. Lament your fate as you're plopped on a tiny island with absolutely terrible spacing for building a base, get rushed by fighters and missile-dropping Rapiers in the first minute, and have to deal with repeat waves of that and hovercraft while hoping you can manage to build up enough defenses and Level 2 tech so you can start building Berthas, Flak Cannons, and Annihilators to finally put a stop to the constant attacks.
Core Contingency's final Core mission, Time to Leave. Unlike most of the Core missions, you start out with a base already built...and also almost completely destroyed, with only a handful of defensive structures and resource gatherers remaining. Within the first few minutes, while you're still trying to take stock of what's happening, you'll be assaulted by a wave of Arm aircraft and Kbots. On the bright side, you start out with a Krogoth...which has almost no health at the start of the mission, and explodes with enough force to wipe out half your base when it's destroyed. Good luck. You'll need it.
