It doesn't require registration. The clients you would like to sync just need to enter the same sync id and password. If I understand right, the password is used to encrypt your data on client-side. The server (called "service" in xbrowsersync lingo) only sees an anonymous sync id with a binary blob it can't read.
If a sync id is not used for a while, the associated stored data gets deleted. That's it. There is no "account" in the traditional sense. I really like this simplistic approach.
It's even open-source.
You don't need to use the default https://api.xbrowsersync.org service but can also host it yourself.
I'm using it and it works quite good. Every now and then sync fails, but I think I narrowed it down: It fails only when you create a bookmark in a just-created directory. But am not sure and didn't file a bug report, though.