these "minor" nerfs are much bigger than you think. The problem isn't quinn as much as its the adc variant of quinn (check out the quinn main subreddit for more detail on what I mean by this). The harrier nerf takes 5 per proc and she procs it 2-3 times a fight. That is 10-15 damager per fight early which is actually a really big nerf to the early game since quinn wants quick trades in lane. The armor nerf also increases the physical damage she takes by around 2%.
The reason quinn popped up in popularity is because of the fact that the crit nerfs and new items actually have little to no affect on her since she is an early game bully that can build crit and do a lot more than other bullys. BUT she has a lower range than other crit based adc's (525 to other meta crit adc's at 550 minimum). Stormrazor also helps her idea of quick short trades giving her a free crit with harrier which hits like a truck to a squishy, which is also losing 10 ad and 5% as next patch. The main reason she is so strong now though is because of the nerfs to the other adc's with crit nerfs.
She doesn't have a stun. Its a knock back that turns you away from her(which has a neat interaction if you time it perfectly with a skill shot making it fly the other way). Her NEAR SIGHT is triggered by being the target valor hits, which a lot less consistent then graves w. Now her ult giving her a ton of movespeed is something people really don't like for unjustified reasons. It is no different from talon e in terms of usability but its her ULTIMATE ABILITY. You know, the ability that can change the tide of a fight or even an entire game. She loses out on something that gives immediate advantage to another when a champion hits 6. She has to use it as a tool to help other lanes instead of helping her own. If you are caught out by this it is your fault, not the design of the champion. You have the knowledge that shes in the game, but YOU do not look or pay attention to the map, which would allow you to see that shes not in lane and probably roaming, or just forget shes in the game entirely.