SyncThing is great when it works, but its platform support is spotty at best (notably lacks official iOS version, I found the Android one to be flaky and if you try to use it on a Chromebook, straight up does not work).

It's great for serverless sync between Linux, freeBSD and Windows machines, though - it generally "just works". Didn't try it on macOS or Solaris, never had the need to.