No traffic ever leaves your local network and since it uses rsync under the hood the devices being sync'd to don't need to run anything other than SSH.
It's a single file shell script that has no dependencies except rsync. It's literally 1,000+ lines of defensive checks and validations to make sure you're not shooting yourself in the foot with rsync, and at the end the last line of code directly calls rsync. It doesn't try to reinvent the wheel by replacing rsync (it's an amazing tool).