Quan is far from easy mode--in fact, against a competent opponent, he can be very difficult to win with. His wake ups suck, everything worth doing in unsafe, and you can't break combos or you lose your offense (rune trap).
Sure, if you play someone that doesn't know how to block mixups, Quan could possibly look easy mode, but hell any character could. Quans mixups aren't even that good.
F1 2, the second hit is low so you can see it coming from a mile away because it's slow
B3 is decent, but there is absolutely no reason why you wouldn't block Quan low
U+3 is ridiculously slow and full combo punishable--you risk the match to make people respect overhead
His pressure isn't great without meter and rune trap--you can duck the second hit of 11, fast start ups can punish in between B3 1+2, and so on.
Winning with Quan, again, against a competent opponent, requires perfect meter management, as well as a few lucky breaks. And in the end, the dash for Rune Trap can be tricky and if it isn't perfect people can jump out. This happens online all of the time, but if you aren't on the dash perfect, it can be dropped offline as well.
I think one of the keys to using Quan is trading Trance from mid screen, though there are several characters that you cannot even attempt to do this with: Rain, Smoke, Kabal are just a few off the top of my head that can full combo punish any blocked trance from mid screen.
Quan is indeed broken because I don't think Rune Trap was an intended strategy, but if he didn't have it he would be at disadvantage in every match up except maybe Sheeva.