Great question! Let me sum it up here for others:
rsync is one-way only and has no memory between runs, every execution starts from scratch. Synchi is two-way, stateful (knows what changed since last sync), and content-aware (uses hashes, so no false positives from timestamp changes). It also handles conflicts explicitly instead of silently overwriting.
That said, rsync is still the better tool for backups and one-way mirroring. Synchi is for when you need true bidirectional sync.
Here is also a comparison with unison and syncthing: https://jakobkreft.github.io/synchi/why.html