I have a C9 and Series X,
I have played with 120hz and VRR turned on and it was ok, the blacks werent as black as normal but it wasnt super bad or anything, still looked "blacker" than my old sony LCD.
However I did have another issue with 120hz, when switching game or going back to home screen maybe 1 time in 20 it would lose connection with the TV and say signal lost, flick back between apps sometimes fixed it but not always.
Currently i have left VRR on but dropped to 60hz and don't seem to have any issues so far with that set up.
My C9 still lags like hell with my sound bar with Dolby Atmos turned on though, I was kinda hoping that would be ok with the new Xbox but really knew it was a soundbar/TV issue where the "passthrough" on Arc is delayed with Atmos, around .75seconds. I can make it work if a go through the sound bar but then i lose VRR and Instant game response so its picture or sound. standard surround is too bad so i'll live with it.