Not OP but the 12.x upgrade has been flawless for me. No issues at all and a very significant speed bump.

For what it's worth I upgraded from 10.11 and had no issues server side, just needed to wait for a client to update.

shows and movies watched long ago suddenly pop up in continue watching and won't go away; used to be able to toggle watched status and they went away, now they just hang there. had to move entire shows out of the library so I don't see them there no more. tiny annoyances that make me go get a mouse and keyboard as they can't be fixed with just the remote in TV mode. nothing's a showstopper but when you're used to the thing just humming along and not noticining it, it's a drag to constantly have to fix this and that.