I do use nextcloud for my small consultancy team but I dislike it immensely, mostly due to personal preferences.
I think I pretty much just don’t like having one platform that does everything. I want to self host my own cloud, I don’t want to self host a thing that provides a cloud.
Additionally, and this really is just a personal preference, I dislike php projects almost as much as Java.
Primarily I use nextcloud’s file sync. This aspect is IMO extremely well implemented. It seems to work very reliably.
We do use contacts and calendars. If there were good alternatives I would switch to them but sabreDav et al lacks a good UI. I’m aware you can disable this functionality in nextcloud.
Anything else that can potentially be integrated is better hosted separately IMO.
Adding to what everyone else has already said, you want sync and backup.
Sync to a central location and backup from there.
For sync, you want syncthing or nextcloud. I would lean towards syncthing for media. If you had a million files in a complex folder structure and a dozen users with different access requirements and instant sync and collision protection is important then nextcloud might be the go. Otherwise syncthing is much more manageable.
My recommendation with syncthing, which is not obvious, is to set up a single hub which each client syncs with. By default you end up with a mesh where everything is connected to everything. It’s very difficult to manage with a lot of folders and devices. Turn off discovery and input the server / hub details manually.
For backup, if you have a lot of media you want deduplication. If yesterday’s backup included ABC and today’s is ABCD you only want to transfer D. This is similar to an incremental backup, but the subtle difference is that with deduplication the most recent backup is the “full backup” with the “diffs” going backwards in time, allowing you to purge old backups. I like borgmatic but there are others.
I would also consider carefully exactly what is worth backing up on what service. I don’t backup movies and tv series at all.
My final recommendation is, it’s critically important to test deploying your backups regularly.