I think the reason many are underwhelmed by the idea of Young, or that he's underrated, is because as a winger he's shown very few signs of development since his move to Villa. He was an excellent set piece taker, a threat carrying the ball at pace, two-footed and had a good delivery in open play from the moment he signed for them.
He added no real subtlety, awareness or consistency to his game since his first full season at Villa, which to this day is still his most productive (in both goals and assists) also. To show so few signs of progression after 3 and a half years at a young, developing team that was progressing itself will cause people to ask questions.
He still plays on instinct, he's shown so little progression in his decision-making it has to be a worry. Wingers will always have that side of their game to them, granted, but it shouldn't be such a major part of their game if they are playing at a top club where you will have much more possession and have many more options than the simple beat your man and whip it in.
That said, I think he's shown signs of progression and a lot more subtlety to his game since being moved centrally. It may just have been because Villa played with such a basic (but effective) style under O'Neill that Young didn't have to expand his game or wasn't given the opportunities to, his game was around being a ball-carrier at pace for quick breakaways and providing a constant source of delivery from wide areas for a very one-dimensional target men. It's hard to play more sophisticated football with Carew, Heskey and the like.