Alexander Larsson commented:
Honestly, I'm not sure that a generic nightly build repo is all that useful. Either you want to run the app, and then you need some level of quality checking (so that it at least start), or you just want some kind of regular build checks and the ability for e.g. bug reporters to test if some change fixed things. These two imho more naturally map to flathub beta builds and CI.
CI generally needs some app-specific tweaks to run the tests and whatnot, so I'm not sure a generic thing for that makes sense.
I think the generally broken state of gnome-*-nightly is proof that nobody is really using it.