The biggest thing I can think of is that X11 is more flexible when it comes to specifying custom resolutions; which some people use to output 240p on CRT TVs for emulation. AFAIK, there is currently no real equivalent for xrandr on any Wayland compositor.
I’ve also noticed that Grsync’s password prompt is broken on Wayland. Thankfully, it has a feature that lets you preview the command it’s about to run, so that you can copy and paste it into a terminal.
Being a KDE user on EndeavourOS, I use Wayland everyday. I switched over a few years ago for Waydroid, but now X11 isn’t even an option for me with the current release of KDE.
Run on older video cards.
My home server has a basic Quadro K420. Since the latest KDE forced it onto Wayland, it’s unusable even for the short remote sessions I need the GUI for. Chrome hangs if I don’t disable HW acceleration, within minutes Firefox leaks memory to the point where the kernel panics, Transmission’s window gets garbled like I’m watching a coded PayTV channel.
Installed xfce and everything is back to normal. Of course nVidia shares some of the blame here, but it’s clearly working on one windowing system and not on the other.