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.
For me it’s Steam Remote Play. I can’t say Wayland is unable to do it, but it’s always been so problematic I switch back to x11.
I never cosidered if that was a problem i had last year when demoing remoteplay onto an apple tv.
I had never thought to check if Wayland was the issue there.
Does gamescope run on Wayland? I’d have thought it would be great for this