This is basically exactly how Android MediaStore API works too: https://developer.android.com/training/data-storage/shared/m...

The difference is that Android also has APIs (which require user permission and are, at this point, mostly deprecated or heavily discouraged through Play Store policy, hence what happened to NextCloud) which offer filesystem-level access to files created by other apps. This has historically allowed for apps like NextCloud and SyncThing to offer automatic backup or syncing.

SyncThing ran into similar problems recently: https://news.ycombinator.com/item?id=41895718