I tried using Mission Control with two monitors for the first time yesterday. Nothing fancy: a YouTube video in fullscreen (aka with its own Space) on the secondary screen, whatever else I was doing on the primary (built-in laptop) screen.

It was such a broken mess. Opening Mission Control and closing it again, interacting with it only on the primary screen, would often (but not always) result in the secondary screen forgetting what it was doing and going to a normal desktop space instead of the video space.

And I'm certain that windows are more prone to overlap now than they were before?