Thing with Hawkeye in tennis is that the camera setup and the way it's visualised is basically perfect. Similarly, with Hot Spot in cricket, the thermal method leaves no doubt whatsoever over ball-to-bat contact.
As Andy Gray was pointing out, with VAR offside decisions there are three key frames that will be used to judge it (potentially two if the pass is a one-touch) which differ enormously. What we're not seeing in these VAR reviews is a 'flick' between the two frames where the ball is on/has left the passer's foot, which I swear we used to be shown before and would likely have left the VAR booth with an embarassing decision to make this afternoon.
Alas, if you understand the technology properly then it's easy to see the potential pitfalls and how it doesn't replicate the accuracy of, say, Hawkeye at all. The mentality that these marginal offside calls being made must all be absolutely correct is actually rather foolish.
At least with the handball decisions they're pretty clear, if admittedly harsh at times.