In a game of poke or be poked now, probably the main reason why Lulu or Janna isnt being played more is because people would prefer using more hard engage to assert dominance... Janna is still one of the best counter engage supports. The shield didnt need to be nerfed really. She was declining due to meta. Also, there is/was fewer marksmen being played which Lulu plays best with. Lulu still has a more overloaded kit than janna because.....
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Lulu Q is a slow able to bypass its own range by having Pix on something else, similar to Taric. This slow is also a very hefty slow on a short cooldown even early game and she can pretty much cast it while not even stopping. Janna's W she stops for at least twice the time and has a longer cooldown and it even removes her move speed.
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Lulu W is either move speed + attack speed buff, or it is a slow + polymorph which pretty much shuts down any damage that the person was trying to burst off and it is nearly spammable at 45% cdr maxed rank. This is similar to mixing Janna's W and a max charged Q knock up on a targetting ability.
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Her shield isnt decaying + it is nearly as strong + pix continues to shoot when the shield is gone for the ADC or whomever... and is shorter cd all through the game than Jannas. Janna gives attack damage boost, but it only lasts until the shield is destroyed. Longer cd. And as mentioned, decays at a significant rate.
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Lulu ulti is a aoe instant hp gain (unable to be mitigated by ignite or grevious wounds), with aoe knockup. This is able to be utilized and abused by SOO many champions. Yasuo can go in, lulu ulties and knocks up, yas ulties. Malphite goes in, knocks up, lulu ulties, knocks up, ori ulties, knocks up and pulls in, all while a MF ulties on them for the entire duration with them unable to do anything about it.... Janna ulti? Knocks back, used for disengage and heals, can only be used as an engage if they can knock the person into their team inwhich means she is WAY ahead of everyone else. Often saves enemy players as much as the allies. Only one who might be able to abuse it is Yasuo and even then it is iffy.
Trust me, only reason why Lulu isnt hit as much as Janna YET is that Janna has an attack damage boost that works with bruisers who often are played in bott instead.
So keep Janna in mind when it happens.