The sync would stall and I'd have to go retry, or it would fail with no error clear message. In the end, I had no idea what had really synched correctly. The app was unhappy if I deleted a photo too quickly.

With Syncthing, I sync to a directory that my Nextcloud user can access (a read-only mount), so I can still easily share photos using Nextcloud, for example.

(although it's unfortunate that the Android syncthing app is being retired. h/t for the heads up and the recommended alternatives)