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.
We don't have an API yet for remote support. Even if we had things like TeamViewer would not work any more.
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.
It's not yet possible to switch modes.
It's not yet possible to rotate the screen.
We don't have an API yet for virtual desktops, so pager is not working
We don't have an API yet for window thumbnails.
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.
QtWayland does not yet support the tablet API. So even if KWin had support Qt would not be able to use it.