Mentally fragile?
Disagree, this is a guy who at 22 has played for 3 big teams in Inter, City (they are a big team now) and Milan and had varying successes at all (early days at Milan if anything he's getting better). Also, he has his fair bit of trophies to show for it as well.
Also, last summer in Euro 2012 he was one of Italy's finest players. And first Italain striker to show up at an International event since Christian Vieri in France 98. Legends like Zola, Inzaghi, Del Piero and Totti have not had similar tourneys.
Finally, his penalty record speaks for itself.
Not the finishes article but a lot mentally stronger than people give him credit for.