Despite the inventors recommend other software I consider distributing my data over various machines in different locations with versioning a viable way to save them from being stolen, corrupted, broken or otherwise lost. OK, I do a manual backup occasionally but syncting does it permanently.
Am I overlooking some cases it fails to protect my data?
Yes. You delete a file, or a disk failure causes Syncthing to think that a file has been deleted. As if by magic, it’s also deleted from all of your backups 
The same happens with corruption: any corrupted files are immediately replicated to all backups.
…and the correct files are in the versioning folder.
Yeah, the versioning folder helps. Bear in mind that old versions are removed from the versioning folder after a time, which could happen in the case of corruption.
I’d recommend file system snapshots instead of (or in addition to) the versioning, on the receiving side. ZFS snapshots, NTFS shadow copies, etc depending on your operating system of choice.
This is exactly how I do it. Syncthing + ZFS is wonderful 
Filesystem snapshots are nice, but a backup which is frozen and remotely stored is a real backup! E.g dedup backup tools like Borg backup, restic.