Because when people criticize him they aren't talking about whether he's good on the ball or not. They're criticizing the fact he gave more penalties away than any other Barcelona player in 20/21, or his clumsy own goals, or the losses of concentration which enabled a player to peel off him and score. Again, you can't judge a CB by stats. He may win most of his tackles, aerial duels & progress the ball well but if he gives away goals then it's a killer.
This isn't just a fan opinion thing, he's been dropped by Barcelona despite them not having particularly prodigious center backs. This is a chance for him to rescue his career & we've given it to him because of a lack of available LCB's on the market. I think there's a high chance of him being a success within this system if he can clear his head. Obviously at Barca at some point he suffered a loss of confidence and hasn't mentally recovered.