I agree with slowness -- I feel the recent versions are pretty fast on Linux, but it can be faster. Also it's "scan, then transfer" approach means it can take lots of memory if there is a lot of files.
Still, if you can your rsync in your application, use it. For automated backups, you cannot beat it. But unison has two unique advantages:
- Proper two way sync -- doing it safely with rsync is almost impossible.
- GUI/TUI which shows what changed and allows conflict resolution.
- GUI which shows