The Shadow of Kyoshi reveals that Kyoshi has a habit of skipping meals. While her Avatar duties certainly play into it, her childhood of nearly starving to death and eating out of the garbage to survive definitely has a huge role in it. Because food wasn't a stable necessity, Kyoshi had to get used to going without food for long periods of time. Now that she's older, it's something that she just simply doesn't think about when she has other matters to attend to.
Yangchen's siding with humans over spirits was foreshadowed all the way back in the original series. When Aang consulted her, her advice was to sacrifice one's own spiritual need for the sake of the world as the Avatar. Now look again at the portrayal of the spirits throughout the series and the revelation regarding her siding with humans in this book. Her advice is based from her life when she had to side with the world (humans) over her spiritual needs (spirits).
Given how Avatars tend to be opposites of their predecessors in personality and approach, this book may explain why the next Avatar appeared to be born to a higher class of the Fire Nation: Kyoshi has no political tact and her opposite would be someone from who appears to be from some class of the nobility (Roku) who'd likely know how to handle internal politics and not be 'the breakdown of negotiations' given human form. Roku specifically tells Aang that he tried to be 'disciplined' as an Avatar, and his greatest mistake (he believes) was not killing the Fire Lord— something that Kyoshi repeatedly threatens to do. And Aang, whose grave mistake was casting away his role as the Avatar that lead to the deaths of his people and his tendency to run away from his destiny in favor of goofing off, was reborn as Korra, who takes the role of the Avatar very seriously to the point of detriment as her ability to identify as anything other than the Avatar. Given how Kyoshi was always hearing about how Kuruk was lazy and feckless (and later carefree and idealistic), it's not a stretch to think that Roku was always hearing about how Kyoshi was violent and overbearing, especially considering that his best friend was the successor of the guy Kyoshi was repeatedly threatening to kill. And even Aang wasn't there to guide Korra anymore, Korra still manages to learn compassion and empathy, two traits that really described Aang's character.
On the above point, who is the predecessor of an Avatar who had to drop a lot of her nation's tenants to be effective (Yangchen?) Szeto, who went all in with his nation's government and became a bureaucrat. His failures as an Avatar, as all Avatars have, was likely focusing too much on his nation's own (severe) troubles and not paying as much attention to the wider world, requiring an Avatar of the world in Yangchen. He also, in doing so, lived to do the will of others, while Yanchen, as seen in The Dawn of Yangchen, had the problem where the wills of other Avatars often overwhelmed her.
Fridge Horror
In the previous book (and all previous Avatar media) it looked like Kuruk was a failed Avatar; all he needed to do was be a strong symbol so that the world could carry on Yangchen's peace, but his carousing and early death ruined it. But the truth is that he was prepared to be a symbol, only to discover that what the world really needed was a hunter to clean up Yangchen's mess. He rose to the occasion and was a great Avatar, but only one other person besides his own reincarnation would ever know.
This also explains why Kyoshi had to be decisive in her role as the Avatar into the point of being Memetic Badass (in-universe or by the fandom). Her past two lives were responsible for the problems that festered in her lifetime. There was no question that she would employ drastic methods and double her efforts to ensure that she would redeem both of her past lives' mistakes.
A sad case of What Could Have Been is that, if Kuruk had more time to live and do things other than 'slowly killing myself and trying to self-medicate', he might have accomplished a lot despite living in the era that didn't have political instability. The guy had insights on bending ahead of his time and was constantly improving, he could have revolutionized bending sooner as fitting of his native element being the element of change, even before the United Republic of Nations (a melting pot of all four established nations) is founded. Who knows what he might have discovered or popularized centuries before Aang and co.?
How many other Kuruk's have their been? How many Avatars considered to be failures were actually keeping the reputation of their predecessors intact at the cost of their own? How many of their stories will be forever lost, especially after Korra Book 2?
On another, darker note: how many other Avatars have died young? There's a lot of them, and as of Korra, the Avatar has only existed for 10,000 years. Assuming an Avatar lives a regular-ish human lifespan of 80 years (on average— Kyoshi, Kuruk, and Aang all mess with this number), that's only about 125 lifetimes. Now, Writers Cannot Do Math, sure, but it makes a sick amount of sense that there might have been an Avatar or two who were killed on an early mission, or murdered to give a villain an extra 16 years in which to complete a plot.
