I wouldn't say he's hard to write. Toph is hard to write extended POV scenes for, mostly because you don't realize how much you rely on visual description and concepts to paint a scene.
What difficulty Aang poses is writing him so that his morality and pacifism isn't a strawman. It's not even that he forgives the Fire Nation for genociding his people; forgiveness has a sort of power to it, that *I* choose to let what *you* did go. Aang doesn't even hate the Fire Nation. He just moves on with his life after Episode 3, and whatever guilt he associates with the Air Nomad genocide is, as shown in "The Guru", the guilt he has for surviving. But it's not really a strong survivor's guilt, either.
Rationalizing that without resorting to hollow platitudes is a real challenge.
Re: Excuse me mum, May I make another
What difficulty Aang poses is writing him so that his morality and pacifism isn't a strawman. It's not even that he forgives the Fire Nation for genociding his people; forgiveness has a sort of power to it, that *I* choose to let what *you* did go. Aang doesn't even hate the Fire Nation. He just moves on with his life after Episode 3, and whatever guilt he associates with the Air Nomad genocide is, as shown in "The Guru", the guilt he has for surviving. But it's not really a strong survivor's guilt, either.
Rationalizing that without resorting to hollow platitudes is a real challenge.