Lux AP CAN be great.
But you need to keep in mind that when you "build" a team in League Of Legends you need to wonder "which role I will have to fufil during a team fight" and "can I actually lane against this enemy".
Lux is mostly poke, thus she can't be used everywhere. You need a team that can dis-engage a fight, else you'll get doomed in my opinion if you meet decent players.
Basically Lux is all about being able to maintain a fair distance from your enemies while trying to catch them with your spells to create a perfect room for engage, or just lower them enough to force them to run away from the fight, leaving you the objectives (turrets etc.)
However, compare the utility of a Lux and Cassiopea during a fight... Cassiopea is NON STOP HUGE damage, both in aoe and on a single target, paired with one of the most game breaking crowd controlling ultimate in the game.
However Cassiopea got "one blow" and needs to pur herself in risk given her low range.
Lux got GREAT range, and well, you can keep spamming untill you finally catch someone off guard.
I took opposite champions to make sure you understand my point.
Basically to help you to see what I mean, during the last world championship we saw alot of Lux.
Why?
Simply because of Jayce, who can dis-engage with his gate.
It also works with Shen, since Shen gives you free time to try to catch people thanks to his split pushing that forces the enemy to send someone defending (so they can't actually start a fight since it'd be turned into a 4v5 in the blink of an eye with Shen ultimate).
That's basically why heroes like Cassiopea, who was so popular and never been nerfed since -iirc-, have disapeared. More and more the ability to engage and create rooms for a perfect engage aswell as the ability to dis-engage has become important.
How can Cassiopea dis-engage? Ultimate? then you're "naked" for the next minute.
Whereas Lux, can just snare and lower the enemy team with her low cooldown spells.
So yes, enough chatting, Lux is good but think about what your team will have to do, how you will proceed during the team fights.
Champions like Cassiopea will carry far more than a Lux in the games you're gonna play. And Cassiopea is actually far better to farm and win a lane given the aggression you can make against your opponent.
So there is no "real better ap" or whatever, it's up to you to make this AP have a great role by figuring out what you'll need to do.
Hope it helped, Lux anyway is seriously funny to play with, she has decent ap ratio, she doesn't "fall of" like someone with poor ratio would, she's just not good during a team fight like Cassiopea, Morgana, Karthus things like that, would be.
But many people think she falls of because in late, there are almost only team fights.
Same as Leblanc, Leblanc doesn't fall of in late, she's just not meant to reach her potential during team fights, even if Lux is more an hybrid between an assassin as Leblanc or Fizz and a "team fight AP".
But if you mean by "fall of" being less good in late and not scale less well with items, then okay she kinda does, but again it depends on your team.
However if your enemies got an Amumu (for instance), it'll be hard for you to keep up because he just wont give a single fuck at you and flash/q then ultimate if he's not stupid, to make you feel the pain to be a Lux stuck in the Melee of a fight you didn't chose to engage.
*Oh, and you should try Nidalee AP, it's hard to pull of since your mates probably will have not a single clue about what it means to play the poke game, but it's even funnier than Lux and it's AN INSANE way to train your own skills (kinda like playing Ashe as ad) since you'll have to take care about your positionning, your mates' HP and your skill shots. Myself I love it, and it's really viable and game crushing against some teams that lack a good way to recover and to engage (yes.. few teams do enter in this case).
