Bi-directional sync is notoriously difficult to get right. See for example the paper "Mysteries of Dropbox" or the contents of any iCloud users address book. I

Bi-directional sync is notoriously difficult to get right. See for example the paper "Mysteries of Dropbox" or the contents of any iCloud users address book. I use Unison which I trust (based on their peer-reviewed publications) and has a good UI to resolve conflicts. It isn't clear to me how Syncthing deals with conflicts or problems.