I won't profess to know his game all that well but from the little I've read - it seems he often overruns to get to the passer and gets beat in the run game because of the QB pursuit. I think another knock is he's described as somewhat "tight hipped". Again, not saying he can't convert but I see him as a part time pass rush specialist that could earn additional playing time as he learns to understand the other responsibilities of the DE position.
To add to your point about his skills dictating a higher draft position than you'd be willing to entertain - I'd add that would probably also ring true for a team running a 3-4. A team running a 4-3 would be interested in him at a higher draft position than a 3-4 looking to convert him would.