For me it is the consistency of when the off field assistants recommend an on field review and when they do not.
Unless the criteria are made much more categorical (eg "did the ball cross the line?") then the technology seems to me to be doomed. Who decides if, for example, an error is "clear and obvious"? How can something so subjective ever be applied consistently? I would be in favour of having VAR for line calls only for this reason.
So the terrible wording of the laws added to human error is as much to blame for how unsatisfactory the technology is. As I have said before, I think this is deliberate. It is difficult not to at least suspect deep corruption within the game.