i use it regularly. it's an extension of cp/scp that does lots of stuff like scanning the directory tree and reading filesizes and datestamps and - i'm not sure, i don't think it includes generating hashes.

IMO, filesystems have several advancements they need but we are still stuck in late 70s unix filesystem designs. A few technologies work with the base and take it further but sadly not enough. A number of things that people associate with Macintosh relate to this. Apple did a lot of effort with making better filesystems but sadly their user-oriented advances have not been widely made use of. It seems to me like, for example, that all files should have a hash in their record that can be quickly accessed. But no. All such systems only create mirror images of the filesystem, if at best, inside a database, no filesystem exists afaik, that keeps a current hash of the file consist. 

 I should just add also, there has been quite a lot of rigidity with this. A lot of "filetypes" actually have filesystems inside them.

Hell, HTML files encode a tree. Hypertext links are like filesystem links across the network.

The technology is still very immature. The benefits of extending the filesystem have yet to be realised, and I'm already 40 years in on this from my early experiences with simple tape recorder soud modulated recording systems, and better ways already existed but just have not been implemented, since at least 30 years.

There isn't enough computer programmers, and partly this is because programming languages are overly complex and build systems unfriendly and labor intensive.