The trouble with Lucien
Lucian has issues with low mana levels and high mana costs, but his issues extend beyond that to a problem of design versus actual play. Lucien is soundly designed, but does not match the way the game is actually played.
His passive is fine in theory, but his mana levels cause there to be problems. Because he needs to cast to keep his damage up using the passive, most of the time he is continually mana deprived. While this can be compensated for by good casting discipline to an extent, it means that he has to wait around for very large openings or ganks, because otherwise he blows almost his entire mana pool in one exchange trying to get off enough damage.
His Q is fine, except that the cooldown encourages spamming and the mana cost punishes it. Most Lucien players end up depleting their mana supply to use this to poke, which seems like it should be the logical use case, but is not meant to be.
His W is problematic. Due to the range and not needing to hit an available minion, it is used primarily as long range poke during disengages. However as designed, he is supposed to hit with it and then use the speed boost to stay in range to continue kiting. In fact, most Lucien players will purposefully HOLD OFF on using W to get in added burst after the enemy leaves AA range. Most of the time it is used, he isn't in range to hit even 1 Autoattack, must less begin a chase. Thus his kit, which needs him to have the speed boost from this skill, is rendered less dangerous by people not wanting to use it that way to try to offset his short range.
His E is an OK gap close or disengage.
R is problematic due to how real people react. Namely, most people can easily walk out of his R. Upon seeing him ult, they will walk, blink/tumble/etc. or flash out of the path, taking only one or two hits. Because it is completely linear and Lucien's movement speed in low, most characters can, if not completely flank him while it goes on, at least maneuver so that they stay out of the ult for the duration even though he can move. The fact that he can move and dash during the ult does almost nothing to help him keep it on target. Compare to MF's ult, which roots her but puts out a hard to dodge cone, which forces the opposition to either rapidly close and escape through the narrow end (If this happens you were doing it wrong) or run far off to the side or back. As long as it is used well, it is very effective at damage and deterrence. With Lucien, you can walk a very small distance to the side and then run directly at him, making it neither good for damage nor deterrence. All in all the ult, while good in theory, in practice is almost worthless.
These factors combine to make Lucien ineffective and unreliable as an ADC, to the point where in 100% of the games I have played with a Lucien on either team, the Lucien has fed to some extent. He is simply not able to successfully engage in lane or in teamfights.