If you need to back up a lot of files, the “tar” method is inefficient and sometimes impractical. This is because the “tar” method makes a complete back up each and every time. It uses up a lot of network bandwidth and processing resources when GPG is used for encryption.
Unison is an efficient method to back up (mirror) a lot of files. This is because Unison only transmits changes. Furthermore, Unison checks what needs to be updated by running parallel scanners on the two systems to be synchronized. This means network bandwidth consumption is determined only by the amount of changes. If a file system has no change from the last time it was synchronized, then very little network bandwidth is needed.
However, Unison does not natively support encryption. In other words, it is designed to synchronize two file systems (local or remote) verbatim.