"-Syncthing is P2P, so you get the disadvantages along with it e.g. all your devices need to be turned on at the same time. If not, you get a desynchronisation between your devices and create conflict.
-Syncthing is not an app for backup (see this link, section 2.2.17). Duple is; in fact backup is one of our main features.
-Then, similar to the likes of Nextcloud, Syncthing also includes a complicated installation/setup that isn't aimed at everyone and requires some IT expertise. One of the main points of Duple is to provide an out of the box tool that anyone can use, with super easy set up.
-It is also worth noting that Syncthing is not planning on supporting iOS platforms."