tarsnap's target audience is sysadmins and other UNIXy gurus. My grandma and my dad, who would benifit most from a secure sync mechanism would probably be unable to use it.
Also, I'm not sure how you can use tarsnap at good cost for p2p sync.
Tarnsap is for backup, I don't think it can really be used for sync in the general sense. It's also hard to predict how much it's going to cost you if you don't exactly know how much data you're going to upload (and is IMO prohibitively expensive for even moderately-sized backups).
I love the tech behind tarnsap, I love that the client is open source, I love the whole philosophy of it but I really struggle with the pricing.
Also, I'm not sure how you can use tarsnap at good cost for p2p sync.
[0] https://www.tarsnap.com/