Oh shit! Have my cries of pain finally been heard? Can you tell me more about this? My last knowledge was that KWin doesn't have pen support because QtWayland doesn't have it and there'd be a GSoC application for adding it.
I use KDE on Wayland. It works pretty well; there's still bugs I hit once in a while (vscode especially sometimes is missing dialog windows and I have to restart it).
I tried yesterday (5.17.3 IIRC, mind you): plasmashell crashed, then followed by systemsettings5, and konsole. Drkonqui popped up for the bug report, but crashed halfway. Then popped up again with a link for manual bug reporting. Clicked it, it crashed again.
Sometimes QML applets crash (there was a bug with the CUPS one recently, if there were duplicated job IDs), and bring down plasmashell with them. But that also happens on X11, which can leave the environment quite empty, and hard to debug for my non-techie family.
On Wayland, I miss primary clipboard buffer (selection), which works on sway. Also, default popup placement is wrong. Besides that, not much.
I really like KDE plasma, but started using sway for Wayland, and stayed for the tiling. I might think about writing a kwin module to ingest my sway config, and only that... :)
At least for Firefox that probably means it's running with X emulation. But the Firefox Wayland backend works fine, it just needs to be enabled. But sure, Wayland still has quite a few corner cases like that.