The Watt challenge is a perfect example of how something can look so much worse when you slow it down, and if you freeze it at the right time it can look horrendous.
The freeze frame that we are all seeing will look identical to a tackle where the player has followed through and caused serious damage, the difference is Watt is already pulling back at that point but you just can't tell from a still image - that's why it's not even been given as a foul in real time then a red after review.
The referee's really need some better training to recognise the difference between the two scenarios.
If the law states none of the above matters and there was simply potential to cause an opposition player serious injury then it becomes a very difficult decision to challenge.