It's UZR/150.
I'm no expert on the formula, but it makes more sense to me, to trust it than observers like us, who watch other players just a handful of games each year.
It measures all the balls hit into a players zone (the "Z" in UZR). Trained observers, who naturally will add a subjective element into the numbers try to determine that catchability factor of each ball hit into a player's zone. Based on the difficulty of the play, players are rewarded more for making more difficult plays than easier ones. Perhaps, because JBJ makes some difficult plays look easier, he gets dinged when he shouldn't, but still, the overall methodology and theory of UZR/150 makes a lot of sense to me. I know it is not perfect, but they rotate the observers to try an minimize personal bias, and all of the observers are trained and cross checked tested.
There's no way I can possibly know for sure, if there are 9-10 better CF'ers in MLB than JBJ. It's just something I can never know for sure. Since I have watched just about every play of ever Sox dame since the early 80's, I can compare him to other Sox CF'ers and say with certainty that he is the best defensive Sox CF I have seen.
Being ranked about 10th does not seem right to me, and UZR/150 should not be the only tool used to rank defenders, but there is a strong chance there are 11-12 great defensive CF'ers in MLB today- just as good or better than JBJ. Saying 10 or 11 are better does not make JBJ any worse than we all think he is. He's a great defender- he just may not be top 9 or 10.