It is apparently a fork from a program called synergy which at some version turned into a paid software. I guess the devs
priority was to make the open source version up to date.
Another big win is the mental order it ingrains in me - e.g. this screen is for sensitive browsing on a sandboxed machine, this is for shells and corporate intranet, that is the gaming box etc.
Also - its "follow focus" (really follow machine) behavior is the perfect sweet spot between alt-tabbing and true follow focus: if I move the mouse to the next screen, I'm focusing an app in that machine. Move back, my inputs go to the last focused app on the previous system. Can't explain this easily in words but it's so good - better than 2 monitors hooked to the same box.
I'll keep a close eye on barrier. I'm happy to see its open source core remain available and maintained.