I like how often people critique nerfs such as the Lee Sin movespeed changes + slight ult CD change, acting like Riot has no idea what they are doing. Lets just ignore that Lee had an almost 3% drop in winrate from those changes.
At least let the changes go through before screaming.
As a side note Ahri changes went like this>
W 10% less AP scaling, across all 3 fires (which hit at 30% damage on additional bolts at the same target). That is 30% less AP scaling across 3 targets, 23% Less scaling across 2, and 16% less Ap scaling vs a single target.
R: 5% less AP scaling, across her 3 bolts, which fire 3 times. Up to 3 bolts can hit one target meaning that her ult caused a 15-45% AP scaling drop depending on the number of targets.
E: 10% AP scaling increase on the ability with 1.5 - 2.4 times the CD of W
In a full rotation of her spells Ahri lost 21% AP scaling vs a single target. In team fights she lost 65% AP scaling lost. Over a 10 second rotation of all of her abilities (So 2 Foxfires) she lost 27-95% AP scaling depending on the number of targets.
Just because it feels like a small nerf does not make it a small nerf.
Think about how frequently Lucian goes OOM in lane, it is very often for an ADC, especially if they spam. They use Q anywhere from 2-5+ times in a bit fight, most bot lane fights early on are 2-3 unless they are very short trades. hitting the mana cost means he can use it less, which means less waveclear and less bullying. That means he can't capitalize are strongly on won trades, and pushing him in hurts him a lot as he has to burn even more mana just to keep up, and as well already covered he burns more mana than just about any other adc.
Hitting him in the mana hits him exactly where he needs it, but hurting his flexibility and limiting his ability to trade all the time. Lucian is meant to deal damage, that is fine. By limiting his mana pool you keep lucian strong in terms of power, but hit his laning phase quite hard and give incentive for him to go down the essence reaver path which weakens his laning phase further as well as focusing him around the mid-late game more so he can't run away with games so early.