Unison is very interesting and it is, indeed, very special in that it solves the very specific use-case of my parent post.
However, my own opinion, and that of just about everyone who cares about backup tools is that 'borg' is the "one true way":
This is not really isolated to borg, so I don't want to pick on them too much (plz shield your eyes in the direction of rclone...), but calling it the "holy grail" is a bit rich IMO. This kind of stuff is simply not industrial grade software.
('attic' on there is actually borg)
Borg has a lot going for it, but I'm hesitant to call something a holy grail or one true way if it can't do compact diffs. Especially when it needs active server software, which makes pruning much easier.
It treats the creation of a new directory as an atomic operation, and rolls it back on failure.
(So, if the initial sync will take a week, then precreate the top level or two of the directory hierarchy...)
It’s an annoying problem, but after the initial sync, partial syncs are fast and reliable.
We try to keep out pricing very roughly inline with Amazon S3. If you know to look for the discount signup rates it should be very slightly cheaper than S3.
We can't promise customers support from real UNIX engineers and also match B2 pricing. I'm happy with that and plan to continue on that path ...
We would be very happy to have you.
"There's exactly one sync program that gets it all right: Unison, written by Benjamin Pierce"
(Ask about the HN readers' discount)