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.
There are a few things that don’t work very well, mostly to do with multi window apps and how some older apps need to run through XWayland (which 99% of the time it works brilliantly! There are a couple weird issues, but most of those are easily fixable through some command argument before opening an app)
I an also running EndeavourOS (which is awesome!) with Niri + Noctalia (which also uses Wayland), and I have no issues :D