Except when they break horribly. Some of them modifies gsettings directly, so even if you remove it, you still have to clean them up manually.

Exactly. I had setup some dock extensions before Ubuntu went back to gnome and I'm stuck with it now, can't seem to be able to switch back. Deactivating / uninstalling said extension didn't work. I uninstalled it years after installing so probably the settings changed name or something.

It's up to developer/maintainer to provide rollback. My experience with Gnome 3 isn't deep because I've switched to KDE after 2 months of tinkering and feel happy ever since.

KDE is kinda okay now, but I just can't forget the nuclear clusterfuck called kde 4 back in the day.

I kinda tolerate gnome 3 (despite its marvellously obvious flaws) for now and if I find it way too annoying, I'll rip it out and use fluxbox or smg similarly low level wm.

Funny, i thought that extensions where "here be dragons" territory where the APIs may break at any moment etc.

Meaning that it is a fig leaf from the Gnome devs to power users.