The window manager now runs on Metal 2, which should give a nice jump in responsiveness, but it is likely responsible for the graphics glitches people have been reporting, especially with older machines.

As a counter-anecdote, I have a rMBP 2012, and I haven't encountered any major bugs (no kernel panics, hangs, lag), though I have seen a few minor glitches (Safari tab bar didn't draw properly, only saw this once). On balance I'm happy with the upgrade however, my system feels more stable and faster as a whole, fixed a few issues from Sierra.