The problem is that this has effectively no conflict resolution. With naive file-based sync the best Syncthing can do is save both files. Then you are stuck merging yourself. Maybe if all of your devices are reliably online and you only use one at a time. But eventually you get a delayed sync resulting in a state fork and you end up with a mess.

For photos and some other type of data this likely isn't much of a problem. But for more complex data it definitely is.