the primary use for syncthing on my phone is to make sure data is backed up immediately.
i have yet to find an app that has the resilience and flexibility of syncthing for this purpose.
apart from that the primary issue i have with syncthing is that the app on a device can not tell me if all data in a particular folder is fully copied to the other device. it only tells me if all local folders are complete.
it also gets confusing when you have more than a few devices and sync between all of them. i recently discovered that i had a set up A<->B<->C but A and C were not connected. i never noticed because A and C were always in sync, until device B died. but it took me some time to notice that because i didn't realize that i missed to connect A to C directly.
it would be nice to have some way to get an overview of the whole graph to see what syncs to where.