For me, it's its lack of proper multi monitor support.
You got your main monitor, and then you got that... Thing. You can put your windows on it. I guess.
Also, windows will just full screen on whichever monitor they please sometimes, the multi-monitor experience is very buggy.
Still on GNOME because I find KDE's config vomit overwhelming for my ADHD brain that just needs sane defaults, but I'm unfortunately finding myself considering a Mac unless another opinionated DE like Gnome comes along.