He is a bit of a dick but that's not that big a deal. The problem is his overall attitude. Rabiot is a talented player who doesn't like to hurt himself during practice or on the pitch. He could be dominant since he's physically strong, he could cover a lot of ground as he's pretty fast, he could play as a DM as he's pretty good defensively. He just doesn't want to. He'll have his moments during a game when he'll look real good, carry the ball and move a couple of players in the process, but he'll also walk when he needs to track back a dodge contacts when he doesn't feel like it.
I don't see how it's going to translate well in the PL. He was barely a starter in the worst Juventus team in a decade and they decided to let him go.
Is he better than McFred? Probably yes, but not by much since he checks out when he feels like it.