Generally not much more then the official releases (occasionally we get a serious bug we missed when merging, but usually those are addressed pretty quickly). In fact a lot of the time the nightly builds are probably less buggy (since we’ve merged a lot of bug-fixes).
As for save compatibility, they are getting better, but we still tend to break save compatibility every few days. If you don’t want to deal too much with save compatibility I suggest that you only update when you get tired of a given world.