Resilio Sync is peer-to-peer, so if you believe their marketing copy, you're not handing your data over to anyone -- it only syncs between your authorized devices. Since Sync is proprietary software, it's a valid concern that Resilio could be making copies of your data without your knowledge.

I use Resilio myself, but I'd rather use free software for this sort of thing. Syncthing looks great aside from the lack of iOS support.