For starters, AUs are "easier" to run under Roestta than VSTs since AUs run out-of-process by default. This means you can use a x86_64 AU without running the entire DAW under Rosetta on Apple Silicon.
I always default to AU just because I’m on a Mac and I arbitrarily decided to do so long ago. The Rosetta thing has been a nice bonus. Not sure what the other (dis)advantages are, these days I try to focus on using native Ableton and M4L stuff anyway!
Yes, this has been my experience with AUs in Logic Pro, at least. You simply get a message saying a plugin failed and that you can try reloading it if you'd like.