You can use it for photos, but for those contained within the photos library that's handled separately through the PhotoKit now. It should be technically possible to backup your photos library through syncthing, I only think the automation of such and the permissions required would probably be annoying to deal with. Not sure if Mobius developer(s) looked into that yet.

If you don't use the photos app, you can keep them all in the files app and it syncs just fine. Annoying to use since the files app is nowhere near as nice for photo browsing/management.