For my part, my criticism of Farrell is much more on the "leader of men" side than the tactical side. Tactical moves are too hard for us to understand with limited information, and since every manager seems to make loads of them, I don't see it as distinguishing.
But, on Farrell's watch we have seen one very expensive player self-immolate (Panda), and another on his way to (Price). While he has had some success with a couple of young players, he has also had a bunch of failures. This is all normal, not all prospects work out, but it doesn't really count in his favor as doing some magic we don't know about behind the scenes. I find the arguments about the team achievements pretty laughable, though. One of those years they didn't win 90 games they went out and spent like they intended to, and the ship crashed and burned. So maybe Farrell is just a front-runner or whatever but I think he's mostly a JAG. It's hard to do better than a JAG and it's really easy to do worse, so I understand the argument for keeping him when things are going well. But I also don't think he's good enough to rally around and oppose a change for change's sake.