Librevault is an open source tool that offers directory synchronization, like BTSync. O wait, BTSync is now called Resilio, for good reason. Anyway, it is geared towards sharing, like Resilio, rather than syncing, like Syncthing, as I managed to pry from the Syncthing developers.
Librevault seems to be solidly alpha software though, so I’m not going to burn my hands just yet. However, I’m happy that somebody realized Syncthings setup just isn’t practical when you share directories with anyone other than yourself.