A few patches ago, Lucian was a very middle of the pack ADC. Sure he was a perfectly fine pick, and was pretty powerful in the right hands, but he was nothing to really write home about most of the time.
Well now we flash forward to present, and he has a pick rate that is exceptional, not only by general standards, but also by the standards of popular champions. Even when a champion is really in the meta and somewhat of a staple, pick rate for said champion tends to float around 30-40%, with 40% pick rate being the limit typically. Well as far as I'm aware, Lucian has made history and set a new record of over 50% pick rate, something I don't think has ever happened, hell his pick rate nearly exceeds his win rate, and that's a bit silly. And obviously, when you have a champion who is both really popular and on average outperforming a lot of the other champions in the pool of viable picks, then you have a problem.
Yet honestly? I don't think it's really down to Lucian. A few patches ago he was in a bad spot and took a lot of skill to effectively utilise. And now he's pick/ban. What happened? Koreans happened (again)
Lucian is in his current state because of the sheer amount of power that stacking penetration offers, no other ADC is performing at his level because no other ADC is building like him. Ghostblade is so powerful due to, not only the extremely powerful active, but the fact that ArPen is an incredibly powerful stat which is equally hard to itemise currently. The fact it gives double the ArPen of any other source of ArPen (Except LW when hitting a beefy tank) is huge, particularly since it allows him to jump on a target and just blow them up because of how strong Penetration is.
And then to top that all off, it's a damage powerspike that is reliable. Lucian will charge in to a fight in the early-mid game because he knows he has the damage to back him up, he doesn't have to sit around and play things safe, wasting his lead on the off chance that the enemy gets a lucky crit, wins the fight and tilts Lucian to oblivion.
Crit is archaic and needs to be transitioned to a system similar to Ashe's passive, and Penetration needs an overhaul, and GB probably needs to have some stats be scaling.
Or just, nerf Lucian. Again. And then nerf GB and/or Cleaver several patches down the line, forcing him back to the previous build and just making him a shadow of what he was prior to the Korean build.