It's supposed to be difficult to summon with 5 different spellcasters.. You can still counter it with floodgate, or monsters that negate effect activations such as Millenium Eyes Restrict
Just saying, Floodgate doesn't outright counter Quintet since you still need to get rid of Quintet from the field somehow, because you most likely won't be beating over its 4500 DEF.
Ironically, Mahad, the original "Dark Magician", actually can tower over this card by having 5000 ATK when it battles this card.
"This card isn't that OP, guys! It can still be countered!"
So can at least half the cards on the limited/semi-limited list. Then again, I guess decks like Six Samurai and Woodland Sprite FTK wouldn't be 'too' broken, if we removed them from the ban list. In one case, you just have to get around all of its control cards. In the other, you just have to... well... 'stop the OTK.'