Nextcloud is more a backup-adjacent system. You can use it for backups, but you're on the hook for maintaining that system and keeping it secure. Maybe you have time and will to do so but most don't. It's a lot simpler than it used to be on Ubuntu (nowadays just `snap install nextcloud` and you're good to go) but that doesn't make it carefree.
I ran my own Nextcloud instance for ~3 years, recently moved to Syncthing for simplicity. But that use case is more about making certain pieces of data available to all my devices, not for backups.
File backup is just one of its many capabilities. I use these apps in Nextcloud currently which sync to all my devices:
News/RSS reader
Cospend like Ihatemoney
Contacts
Calendar
Music
Mail
Photos
Talk for voice and video
Bookmarks
Deck/kanban board
Tasks
Notes
Maps
Polls
Forms
Money
Health
Passwords
Collectives/Wiki
I did the same with my instance. More power to you if the tools are good enough for you, but I found them too clunky to use compared to dedicated products in the space.
Still, I did appreciate the breadth of apps that one could install.
Nextcloud ecosystem is best of class rather than best of breed. Not every app is the best, but many are under active development and improving rapidly. I might have too many eggs in one basket, the the maintenance is very easy this way.
I ran my own Nextcloud instance for ~3 years, recently moved to Syncthing for simplicity. But that use case is more about making certain pieces of data available to all my devices, not for backups.