Even Riot thinks that she sucks...
Her design is just broken.
Valor is just weak, melee as adc? Kinda insta death. Execute dmg? Kinda ok, but because its AOE its super weak.
Even base dmg from graves ult deals more dmg without the need to look for the execute dmg.
(Comparison: Graves: 250 / 400 / 550 (+150%AD) (without explosion) and Quinn: 100 - 200/ 150-300 / 200-400 +(50%-100%Ad)
An adc needs to do dps over time, staying safe and survive, her kit just does the opposite.
Her best steroid is her ult, but she cannot use her ult because she would die in a blink.
Her passive is to random to count on it and the base stats are too weak.
She kinda excels as lane bully on the toplane and is a good duelist, but as soon as laning phase is over, she just falls into an endless pit.
She cannot go offtank, because after the aoeblind (which is good for autoattacker, like caitlyn and yi, but bat for ad-caster and mages) she just offers nothing a normal off-tank can't do better
She kinda is designed as a ranged assassin, with similar problems to talon, just even worse. She can get in, but she wont be able to get out anymore, as she needs her ult to deal some dmg boost.
Riot once release a test on the pbe, where they lowered the cd on the ult by a ton, so she can get into the fight and out of the fight.
But kinda never thought about her again.
tl;dr
It is not to just offend any Quinn players, but her kit counters herself.
The passive can't be controlled well enough and the aoe execute is too weak.