I recently backed up and restored my main home server so I could reinstall it, and ran into an issue with the user and group IDs having changed -- which made restoring more complicated.

A writeup on the small script I wrote to fix the situation: https://www.brainonfire.net/blog/2020/12/27/rsync-auto-mapping/

Of note: It was very, very useful to have backed up /etc/passwd and /etc/group along with the user data!