As others have said, sync isn’t backup.

It may be part of a backup plan, however.

I use Syncthing on my mobile devices to keep data created on the devices synchronized to my server at home. Things like photos sync to home over any connection, while I sync other stuff only over wifi. Syncthing-Fork allows you to set these conditions on a per-folder-pair basis.

That server becomes my authoritative box for any data. All that data is then mirrored on a schedule to 2 other systems at home (a NAS and a large drive on another box).

The main server also has a cloud backup which runs continuously.

So I have 3 local copies of data to recover from if I have a hardware failure, and a cloud backup.

I find tools like Syncthing and Resilio are good for synchronization, especially mobile devices. But between full-pc-OS devices, I just use native tools (scripts and schedules) because I don’t want synchronization, but specific patterns of copying/mirroring, etc.

I do use Resilio for ad-hoc access to almost any file on my server, since it’s Conditional Sync feature permits me to connect with a mobile device from anywhere and sync only the selected files. So I can grab a movie or TV show, Resilio will sync it and I can watch it once the sync is complete.