This is based on two players sprinting in the opposite direction and 50 FPS cameras. A) How often do we see this happen? B) How accurate do you think a linesman's call is in real time in this scenario?The worst thing is that due to the slow camera frame rates in use, it's possible that a defender running one way and an attacker another, the distance between them can be 30cm per frame.
Even with the limitations of the current technology, it's 100x more accurate than a human making the decision in real time. I've seen a few people claim that VAR are actually using much higher quality cameras than we're aware of but there's nothing concrete on that however one thing is for certain, they're coming and any margin of error will be much smaller by next season.