Again comes down to the rulings more than anything I think?
The assistant has to make a split-second decision on how sure they are about an offside and if they think it's obvious or now. The game is so fast-paced these days, at an insane level. Essentially the reason why VAR comes in is to support the officials, not necessarily take the trust away from them.
How many times do we see it on TV (from a different angle, can be good/bad though) where we say 'no way, he was onside!' or 'that's definitely offside" and we turn out to be (clearly, sometimes) wrong. Same for assistants and yes I know they're professionals and have the training etc but it still doesn't make it much easier when it comes to how quickly they have to weigh up the decision.
I can see why the rule is now in - assistant's can even put the flag up but the referee doesn't have to whistle if the player is in on goal. It takes away the doubt of the goalscoring opportunity and gives the attacker the benefit of the doubt. How annoying was it when a player was through on goal, yet despite being onside the flag went up? The new rule enables that chance to be completed and checked later.
My only real issue with VAR isn't anything to do with what it checks but purely the time it takes to check something. Having said that, even the timing has improved since it first came in and most decisions in PL have been relatively quick.