E is the biggest problem. It's one of the most anti-melee abilities in the game and it's extremely easy to use correctly. Cait can do something similar, but she can get away with it because it's a skillshot that she can miss or end up wasting at the wrong time. Not until Kalista has a ranged champion had an easier opportunity to completely negate the agency of a melee opponent. And Kalista was dumpstered specifically for that reason.
This has been a problem for years, but Riot never did anything because Quinn top was a rare pick. Quinn was allowed to have this incredibly toxic laning phase simply because players almost never had to deal with her since she far from popular. (Although that seemingly never stopped them from slapping Yorick around with the nerf bat before his rework)
Unfortunately for Quinn she's becoming more popular, and her greatest defense against nerfs is crumbling because of lethality. More and more people are realizing that hey, I can almost effortlessly hard counter 90% of all melee champs in lane with one pick.
To be fair, Quinn has her own problems. She's got that "Win lane, lose game" disease since late game the Quinn becomes the team's B-grade second marksman and her laning opponent usually buys tank items since they're behind and becomes a meat shield on the frontlines. One of those is a lot more useful to the team than the other, especially in the late game when marksmen tend to evaporate if they don't have someone beefy standing in front.
Although even that is becoming less and less true as games are often being decided before the late game these days. Quinn has got it pretty nice right now, which means melee champs in top lane don't (think like when Gnar is strong). I suspect Riot will be forced to deal with her if her popularity persists after lethality is brought down to sane levels.