My big issue with the ruling is that for me the change in direction and height is minimal. Tackling like that Farrell would very likely still have caught him high even without George's interference. The argument is that the players have a split second to tackle and change and therefore Farrell could not have adjusted. Issue is, as highlighted in the guardian article, if that is enough to justify only a yellow, then what else would be? The player swerving slightly to the side? A slight bend in the knees before impact? If George's interference is enough then you can start to find almost any mitigation for tackles like that as the defender is always moving.
For me it's quite simple. Farrell is making an illegal tackle. He's not bent at the hips, so regardless of what George does, Farrell was always going to be high. This nitpicking over slight changes in movement means that players will take more risks to tackle like Farrell does. It's a slippery slope and a bad precedent.