Not an expert by any means, but first of all I’d say everything depends on your specific needs. File versioning built into Syncthing is a type of backup, but it’s mostly limited to recovering single files with no advanced functionality (unless you use external versioning with your own custom scripts). For example, if you need to recover versioned data in full (e.g. a whole folder tree) to a specific point in time, then it won’t be very useful. As for “proper backup tools”, you may want to check this excellent post at https://forum.syncthing.net/t/ignore-delete/15414/22 by @imsodin which lists a few specific recommendations.
The main point though is that without file versioning set up, Syncthing isn’t a backup tool at all. It’s just like having a RAID1 array. It will protect against sudden hardware failures where the whole drive goes kaput, as you can then recover the data from other devices, but it won’t do much more.
That said, I personally find Syncthing’s file versioning extremely useful, especially since it’s easy to set up and simple enough so that even a non-technical person can use it (in contrary to more specialised backup tools).