I used Unison many years ago, and it worked perfectly for all my use cases. Not sure how it stacks up these days. CLI only IIRC.

I still use Unison as it is simpler than Syncthing.

It does have GUI, which I use. I wouldn't call it pretty or polished, but it works and I understand how it works and the way it works is exactly how I think syncing should work.

I've also configured it to run a GUI diff tool diffuse to easily combine changes in case of conflicts (when a file was changed on both sides since the last sync).

I wish it was a bit more modern and re-written in a modern language, but that's secondary qualities for a program.