Leaving aside the fact that Tarsnap is based in Canada (since really, I doubt CSIS has any more qualms about asking for data than the NSA does), the whole point of Tarsnap is that I can't reveal customer data because -- unlike Dropbox -- data stored using Tarsnap is encrypted using keys which only the customer holds.

Are Syncthing or Tarsnap US-based? No.

And Syncthing is just a tool that does multi-way sync across devices you own. If a company wanted to replace Dropbox with Syncthing, they would have to run one or more servers to provide the "always on" feature that their employees' "sometimes on" laptops sync to.

In terms of this one concern, more obscure services probably are safer than one which is so overwhelmingly popular, and it makes a difference if the service does not have your data in plaintext.

Of course it's easy to give up other kinds of security in exchange for reduced profile to NSA, that's a whole other issue.