You can argue that it is harder to make judgement calls (which officiating inevitably is a match is) when your judgement is being called into question by someone else. This is what is different when VAR enters the equation versus officiating without VAR. I am not a psychologist, but I would assume that self doubt creeps in every time you get the VAR team in your ear, and the fact that he is forced to look at the situation on a monitor might explain why Kavanagh feels that he has to disallow the goal, even though it is not in line with the other calls he makes in the game.
I believe that might be the single biggest problem with VAR.