For me, the plasma 6 implementation misses nothing. Multiple monitors work with no issues, and every program I could run works with no issues.
My main problem is that none of the tiling wayland compositors ( hyprland for example ) work well with multiple monitors. My usecase is to keepcmy laptop's monitor in clamshell mode and just use the external one, but I tend to if I leave for a long time to turn off the monitor since plasma can't turn it off the output for powersaving by itself for weird reasons and plasma 6 kwin will corectly start up on the monitor if I turn it on.
Compositors like hyprland for soke reason won't and will ontly show blank screen and not even allow me to change to another tty, effectively freezing my system.
But I got used to the way plasma works, made it work similary to a tiling wm for the virtual desktops and placing speficifc windows in specific virtual desktop and stuff like that, so I get the benefits of a good stacking (floating) wayland compositor with robust virtual desktops support.
Weird, I've been using hyprland on multi monitor for a while
Only issue I've ever noticed is that some games will insist on running on my second monitor for some reason and will stay locked at at 1080p when moved to the higher res one
My problem is I only use external minitor and turn off laptop monitor, so when I also turn off the external monitor and then turn it back on hyprlamd just has a stroke.