The problem is the UI won't be smooth on VRR displays unless they implemented some clever workarounds. Your mouse cursor will only update at the same framerate as the rest fo the game. Even budget gaming monitors these days support g-sync, freesync, or both, so it's no longer a problem that only affects a small % of users with deep pockets.

The only workaround I can think of that wouldn't also defeat the point of having VRR would be to add some in-engine low framerate compensation artifically boost the reported framerate to a multiple of the real framerate, but I have yet to see any games that actually do this.