Well I can't really thing of many young keepers that I can say are the best in the world U21. But the guy you named would be up there although I'd have to see him more.
Buffon and Dida are both better than Casillas IMO. He is a class keeper but allround I trust them more.
No keeper is perfect at the end of the day so any keeper can be torn apart by critics,Buffon is already a legend even if he makes howlers now until he retires.