You're saying Leblanc is fine and then bitching about Lux in the same sentence.
Do you even play this game?
For Lux to actually burst anyone dead, she needs a ton of items against a champion with no MR. Her abilities can be dodged (they actually nerfed the size of the E so it's easy to dodge now), and unless she lands the slow moving skillshot snare that can be stopped by minions, she's not killing anybody. She's also immobile.
Whereas Leblanc doesn't need items to burst squishy champions down, she only needs them if she wants to kill the tanks, and even against them they won't survive unless they're ahead and stacking MR. She can't miss her abilities unless she's inting (technically chain is a skillshot, but she blinks into point blank range and it's unmissable then pops back to her distortion point so she can't get hit in return), and 1-item LB still outdamages 2.5 item Lux by a fair margin.
There are some items in the game that have magic resist tacked on to some offensive/utility stats. 
are among them. These items do plenty to stop Lux from killing people with her burst combo. Leblanc? Not even phased. The damage is so much higher from Leblanc that such items' minor amounts of MR aren't even a speed bump. To actually survive LB burst, you need full tank items and multiple of them. There's no way for any champion not designed to be a tank to feasibly not die to her. Even sitting in the fountain isn't safe because Leblanc will be back out of the fountain before the laser kills her.
Want to gank a Lux in mid lane? Send the jungler, if she's pushed up she dies and/or has to burn Flash.
Want to gank a Leblanc in mid lane? You need every item on this list:
- A member coming from her left and another coming from her right
- Her opposite laner in lane and full health with summoner spells available
- A member coming from behind her own turret
- Point and click hard CC
- Ignite to find the real Leblanc
- Ten wards around mid lane because if Leblanc's jungler shows up during the gank your entire team dies, or the jungler has to be dead
- Stopwatch, Zhonya's Hourglass or Guardian Angel on every member coming to gank so she doesn't get a kill in trade
- Top lane pushed into turret with Leblanc's ally in top dead or in base without Teleport
- If any other member on Leblanc's team has Teleport taken, it needs to be on cooldown or that member needs to be dead
- Some luck on your side (even with all these things, she'll still somehow get away or get a kill back in trade without some luck)
Even against a mid laner who has a way of playing against her (there isn't one, but let's pretend), she'll just roam bot and kill the bot laner. Even if that bot laner is under their own tower with a support beside them, and Leblanc's bot lane and support are in fountain without Teleport. Then she comes back to lane fed and dives you under your tower over and over again until your team reports you for intentionally feeding.
