The most likely problems are simply dependencies getting out of sync, which results in a) updates not applying and b) an uninstallable nightly state — not actually a broken system.
We're working (slowly — we've got a decade of inertia) on an automated system to move updates which break this consistency to a "side tag", so they can be worked on without breaking the main tree. And possibly in the future we can actually have a similar thing with automated functional testing (but... don't hold your breath). (Also, shout-out to openSUSE, where they're already doing a lot of this.)
Of course, as with any "rolling release", you have "oh look, the UI of my software is different today".