Lux is in an interesting place right now because she’s not necessarily weak, but she isn’t exactly the strongest either. She has some pretty obvious strengths and weaknesses - amazing utility with all her CC, AoE shield, and an amazing low cooldown ult, but relatively long early cooldowns, high mana costs, highly immobile and a clunky passive.
I think the biggest issue I have with Lux is that everyone single one of her skills requires her to stop moving in order to cast and for a mage who’s already immobile in the first place, this is a huge crutch. She has no effective way of getting away and her shield is useless when she’s being chased (mostly due to the small amount it shields her, for the long time she has to stand still to cast it). Basically, she has very unreliable disengage and positioning is HUGE for her.
My other issue is with her passive is practically useless out of the laning phase. I think it should either be able to be procc’ed by allies or at least do something more, like grant true vision. She technically shouldn’t ever be in range to proc a passive with an AA in teamfights, but that’s also a huge chunk of her damage gone.
I think Lux is just in general, a little outdated. I don’t think her kit needs a rework, she just needs small tweaks like updating mana costs, etc. She’s not played as often simply because there are other champs who are so much easier to play in terms of positioning and give much less migraines when it comes to mana.
Also, support Lux has 2 issues - mana and her ult. She’s too much of a mana whore to be effective in bot lane and her ult brings no utility to the team. Her AoE shield is cool, but the 35% AP scaling isn’t all that great, especially compared to Janna’s Eye of the Storm which gives you 240 health (which is 60 health more than Lux’s) +70% AP scaling and an AD buff (or even Karma’s shield). (Yes, Lux’s shield is AoE, but we all know how our teammates all turn into Faker whenever we try to shield them.) Plus her shield is a skillshot, not instantaneous one like Janna’s or Karma’s.