Extensibility ~= extra complexity will necessarily increase the chance of bugs.
In something as complex as a display stack this is an important tradeoff.
This is only true if complexity under the hood actually affects your default experience. I don't think it's the case for KDE. "The chance" is indeed higher, except in GNOME it seems the bugs are actually real.
> I don't want extensible software. KDE is terrible in that regards. They have miriads of options, that's too much for me.
Why not use the default provided then and take the defaults as opinionated? That's what I do actually. I might change very few options, but I generally use the defaults. It's not that you have to configure kde before it becomes usable, the defaults are pretty ok.