There’re discussions to drop the X11 backend with the release of GTK 5. That’s still many years away and I really don’t think there’ll be much of reason left to use X11 by this point.
What is actually still missing for Wayland?
Absolute placement of multiple windows for some scientific applications (multi-process, multi-window apps are places arbitrarily on Wayland atm, excluding compositor specific solutions).
Proper colour management support
VRR working while the cursor is shown. Needs hardware cursor (?) support in the kernel and drivers. FPS games usually don’t show cursor, so VRR works in the games which benefit the most from it.
Both are likely to get fixed in the coming years and are pretty niche.
Obviously I’m excluding compositor specific issues, like VRR, server-side decorations and global shortcuts not being implemented on Gnome. Generally they would work, if implemented.