I use KDE daily, including a multi-monitor setup, and it is not buggy in my experience. Like, at all, much less "very, very." I wonder why we have had such different experiences? I'm using Fedora.
"very very" was a bit exaggerated but it's far from perfect. There's always something. I'm using KDE neon which should be as new as it gets. For multi monitor: it forgets that I had a second desktop, renders it black, switching between two monitors and none is also error prone.. it's not "bad" I'm used to it but as I said, some bug been here for a while now.
The black desktop used to happen on every brand of GPU (I have a mix of Intel and AMD), it just had to be the third or the fourth display, something like that. Or after sleeping and waking. I used to have it a lot.
I haven’t seen it in a while, though, so it just might be they have fixed it. Fedora + zawertun COPR, for what it’s worth.
I recently fixed this issue with KDE not coming back from sleep by setting it so that the VRAM is all saved on my nvidia card: https://nixos.wiki/wiki/Nvidia
Trying to explain it I'm actually not sure I 100% understand the solution. It looks like it uses systemd for sleep AND saves all of the VRAM (I'm guessing into RAM if I'm using anything other than hibernate).