I would argue that gnome is pretty stable in recent years. Don’t remember when was the last time something crashed.
This might would probably be true for Extensions.
KDE has been unstable for me on Wayland in the past.
You’re talking about two different kinds of stability. They are talking about development stability. You are talking about runtime stability.
One thing is to not break applications that use your library because of changes you introduce to it. Specifically changes that go against the standard you’re supposed to be following.
Another thing altogether is to not go outside the memory limits of the application so it doesn’t get yeeted by the kernel.
I’ve had growing pains for KDE on Wayland in the past but it’s been chill in recent times too, I can’t remember having any issue related to that for a long time
