Contrary to what many people seem to think, the FA Cup does not start in January. It started back on Friday 6th August at a handful of lowly non-league grounds. Even moving forward 3 months to the start of the Competition proper, this started at Sudbury's modest, but well equipped for their level, ground with their 3G pitch. It is unrealistic to expect all rounds to be treated the same (unless that means no VAR whatsoever).
As for treating all matches in the same round, the same ....... why? Fairness is both teams being treated the same in each game. It seems perverse to not use something which is designed to improve fairness, as much as possible.
(The fact that some of the decisions made by VAR are staggeringly inept anyway is a completely different issue.)
Indeed - as I've said before, if you're arguing for all or nothing in the FA Cup then you're really arguing for nothing. It simply isn't viable to use VAR in every single match from August through to May.