When do AUs work? When should you just leave well enough alone?

I had a thought today, while I was tearing shelving apart.

I started poking my nose back into Marvel Comics fandom this week, after an extended absence, and basically started with porn, as you do.

I'm not sure I'll ever be comfortable enough with comic canon (with the possible exception of something like Ultimates, where the canon is much, much shorter) to write “real” fic with a plot. I am, frankly, terrified of comics fen and of getting it wrong, and I'm reasonably content to crank out porn or gen ficlets for prompts and save my plot ideas for canons that I feel more comfortable with.

However, I had this thought, and I don't have time to go googling around. Does anyone write/has anyone ever written truly AU comic-fandom fic? ( blah blah and blah some more )

Basically, I don't see any reason why one would need to write an AU, and I can easily imagine some problems with writing one (which may or may not also be reasons why the stories would be unpopular). Being/becoming a superhero is a transformative experience. Are there characters that you think just wouldn't work anymore without their superness?