Joe Hart is a lot like De Gea in that his shot stopping is really excellent but he can be a bit iffy in general play, especially flapability on set pieces and dodgy positioning from time to time.|
The difference is Hart is English so his excellent shot stopping ability gets translated as 'BEST KEEPA IN DA WURLD' and his vulnerabilities get overlooked (despite there being absolutely nothing wrong with saying he isn't the finished article yet). With De Gea his excellent shot stopping is begrudgingly given to him but his other vulnerabilities get translated as 'will never amount to anything cuz hes shit'.
To call Hart world class you're putting him alongside the likes of Buffon, Casillas, Kahn, Zoff, Schmeichal, Van Der Sar, Yashin etc. It shouldn't really be a point of contention that he isn't there yet.
It really is possible to be better than some and not as good as other in football. People always want to go to one extreme these days. IF you don't think they are the best in the world then you must think they are shit!