The Nextcloud app has been running and syncing in the background on 3 iPhones for like 6 months, so they managed to make that work relatively well.

My issue is other bugs that make it painful, including the fact that I cannot trust that Nextcloud will eventually upload the whole photo gallery (it seems like some files regularly get "locked" w.r.t. "webdav", for some reason, and this never resolves).