He does have flaws to his game, as are always mentioned, but I do think that he is so good on his line that it does mask a number of his weaknesses.
When you're as good as he is from inside the 6 yard box, coming out and claiming the ball or sweeping up isn't as big of a deal. Similar to a completely one footed player who is so good with their one foot, their reluctance to use their other is hardly noticeable. Paul Scholes had a naff left foot, but he could pass better with the outstep of his right than most could normally and his shot from 25 yards without taking a first touch was better than anyone, so it was barely a mark against him. In nearly all situations, I'd prefer De Gea in his 6 yard box than anywhere else. That's where he is better than anyone, and whilst there is more goalkeeping than stopping shots, that is the most crucial part of it. If a goalkeeper is going to excel massively at something, make it that.
Whether he is among the best around doesn't really matter, but to most United fans he is purely because of how often he has kept us in games through saves that look impossible. I struggle to name a goalkeeper capable of the same, or one I'd want to swap him for, certainly not until we have a non-calamitous back four and a defensive midfielder who doesn't operate in a slow motion. I think we're a few windows away from that.