Ignoring Flap, megacollabs where every part is really short. You can have megacollabs with a few short parts and it can still be fine, but levels like Flamewall or even Vortex of Time are just fundamentally broken. The player doesn't get time to process or pay attention to what they're looking at, and the creators don't get the space to flesh out any ideas. That's why the best part of the latter is the endscreen, it actually has the time to do something instead of being a mere blip.

Flap gets to be the exception because idk man it's funny