I decided to try Nextcloud exactly because of this. My problem with it is more that the whole thing is a bit unreliable. Like once in a while the app will get into a state where the only way I found to recover is to just erase everything and re-sync everything. And the app will resend ALL the pictures, even though they are already on the server.
And I can't do that with my family members' phones. It doesn't matter to me if the app takes a month to sync the photos, but it has to require zero maintenance. I can deal with the server side, but I need it to "just work" on the smartphones.
Searching for "nextcloud ios background sync" shows a whole bunch of forum posts and bug reports about it not working well unless you have the application open.
One issue (https://github.com/nextcloud/ios/issues/2225) been open since 2022, seems to still be not working properly. Another (https://github.com/nextcloud/ios/issues/2497) been open since 2023.
For something that works well it seems like a ton of people have a lot of issues with it. Are you sure you're on the latest iOS version? Seems like people experience the issues when they're on a later version.
Android is more relaxed but the vendors (like Samsung etc) will go around that and implement their own aggressive background killing bots. Sometimes, this causes alarm apps to stop and not wake you up etc.
The main reason is battery life. Tragically, this makes sense due to the cesspool of spam apps that plague their ”curated” app stores. If you’re an app developer who want to use it responsibly you’re in for a world of trouble. I know because I am one of them (well, I consider myself responsible at least).
I think that the forum posts may be old, and/or a bunch of them may come from users who did not know that they had to set the location permission this way (which admittedly is unintuitive for photo syncing).
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).