I keep a list of cloud sync solutions I've looked at for a similar use case: https://github.com/pjc50/pjc50.github.io/blob/master/secure-...
I'm still not quite satisfied either.
Oh! You were looking for audited sync clients.
Yeah, I've got nothing there.
Edit: I would ask "How hard could it be to solve 90% of the problem?", but various BigCos have had spectacular failures in recent memory, so I guess the problem is pretty damn hard.
I wonder how terrible using git as the backbone for one's sync software would be.
There's git-annex for that. I've not tried it myself.
I've not found an open source tool that does block level backups with diff/compression support. Getting the client experience right is hard and not a whole lot of fun, so I'm guessing there's little incentive for companies to open source that valuable part.
Thanks for the info on Erlang.