I think he's suffered from not having an obvious stand out attribute. He's not amazing in the air, he's not got particular pace, he's not as good on the ball as Stephens etc...
However, he does seem to relish the art of defending and he's got less visible qualities like grit, commitment, application etc. Basically, a bit like Fonte. He was never the best at anything in particular but he still added qualities to the team.