This page tracks the Wayland showstoppers through out the stack
The NVIDIA blob is not supported as it uses a custom EGL extension. It would require additional code just for NVIDIA. On the other hand many users are on NVIDIA. Further information: To EGL Stream or Not
QT_QPA_PLATFORM is set to wayland breaking any application which has Qt included (e.g. appimages, proprietary software) and doesn't have wayland QPA.
No remote support
We don't have an API yet for remote support. Even if we had things like TeamViewer would not work any more.
No (wacom) Tablet support
KWin lacks integration the tablet support from libinput. Also KWayland lacks wrapping the API for it.
We lack a functionality like XModmap to remap keys.
Modes cannot be configured
It's not yet possible to switch modes.
Output cannot be rotated
It's not yet possible to rotate the screen.
We don't have an API yet for virtual desktops, so pager is not working
No window thumbnails
We don't have an API yet for window thumbnails.
No window move on empty area in breeze
Broke with switch to XdgShellV6. Needs new API added to Qt (native interface), KWayland and Breeze.
The multi-monitor experience on Plasma Wayland is lacking compared to X11. This is a problem in particular for those who often attach an external monitor to their laptops.
Activities are probably not a must-have feature, but anyway this is annoying for those who got accustomed to them on X11.
No animated cursors
Key repeat not working
Compose key not working
No tablet support
QtWayland does not yet support the tablet API. So even if KWin had support Qt would not be able to use it.