> Not knowing what a software might be doing is problematic but isn't it the standard?
This sounds so bizarre after using Linux for so long. Most of the software I use is indeed FOSS.
As far as BitTorrent Sync alternatives go check out Syncthing[https://syncthing.net/].
Another problem that I have with Syncthing is usability. With BTSync I can be pretty sure that if I send a sharing link to family, that they will be able to install the software and use the link. Syncthing would be too difficult to set up for most family members or friends.
So, if you are looking for syncing some machines on a network under your own control, Syncthing is great. If you are looking for a Dropbox replacement that works everywhere with good sharing capabilities, BTSync is clearly the winner of the two.