I don't think he's going to thrive in the PL. He's already had plenty of chances to show what he is in the PL. He isn't going to be a disaster, but he'll be the same goal every other match player. Which, as you say, isn't worth £100M. The reason they gave up on him before is he wasn't a golden boot contender, and he still isn't.Lukaku is a good striker. I don't think he'll be a disaster like that.
He's just not the best striker going, and he's not a 100m pound player.