Rebounding is a relative weakness. It's a little tough to tell how much tho, because a) he's always played opposite elite rebounders and b) since he's versatile defensively, he's often not under the basket when the shot goes off, as he switches on almost all picks/screens, meaning he's on the perimeter more often than your average big (but for the right reasons). Kentucky has been the best rebounding team in the country the past two years -- it's hard to imagine you could do that if your center was a liability on the boards. Still, it is very fair to say he hasn't shown great rebounding skills. I don't think he'll be terrible at it myself, but probably no better than average for his position.
Also, he's underrated offensively. He's added more to his game every year -- definitely someone who's worked hard to improve. I think thats important, because he's a player who's ready for an NBA role right now, and can add value on a team, but is still growing as a player. He's got upside, but also a very high floor for a big, as bigs always seem to be very high-variance picks.