Well, you rely on third party devs. So there might be conflicts or breakage when something updates and something doesn't. Not sure how it is in reality, but this gnome stubbornness is quite off putting to me.
They're not that common. In my experience a highly extension-ified gnome still manages to be simpler and more stable than KDE with all its native customizability.
I wonder what's everyone doing with KDE? I've been using it for over 3 years on my PC and it was always rock solid for me. It's openSUSE Tumbleweed with all AMD build.
Truth be told I don't customize it heavily. Just minor tweaks here and there with the rest being default, which I find good enough.
I'm also running KDE on arch. It's not so unstable that it crashes, but its features do tend to break. Right now, there's an empty space in my top panel where the native system monitor should be doing its thing. It was working a couple days ago, now it isn't. Yesterday I found a stray native media player widget on my desktop that definitely was not there before. I had to restart Spotify for the 3rd time today because its window becomes unusable if it's left in the foreground when the system goes to sleep.
I didn't do any deep tinkering at all. Vanilla KDE plasma 6 where the only tweaks I have made are those offered by the DE itself. I'm not impressed.