Ancient applications that were abandoned long ago, hardware incompatibility, outdated drivers, lots of stuff from shifted to Play Services that are missing or borked in AOSP now, etc.
Shit, most of the time notifications don’t even work.
Google got what they wanted from AOSP. They bought it, they got everyone onto it. And then they started killing it and shifting everybody to use play services and be more beholden to Google. You can never trust big tech.