ADCs are not overpowered
That being said, many of them are currently extremely obnoxious. Which I mostly blame on certain support champions, two particular enchanters, as well as how abusive kleptomancy has been, rather than on the ADCs themselves.
makes any assassin that's not Nocturne hard pressed to be relevant in helping take down the ADC, even with edge of night. The extra health and knockup from her ult, her shield, and the targeted point and click CC from polymorph, just makes it incredibly hard to take down your target.
Even with double spellshields and a complete lethality build, it can still be extremely difficult to take out the enemy ADC if Lulu is babysitting them. The double spellshields just makes you slightly less likely to get immediately nullified and killed.
like Lulu, also has two hard CCs, a slow, a massive shield, but has an even more impactful ult if you're unable to interrupt it quickly enough.
The biggest problem with Janna, is that she gives free AD. A ton of it. Max level shield with a measly 200 AP is a 350 HP shield, not counting the major boost from ardent and redemption, and 60 free AD. That's more free AD than a max level Vayne ult gives after her nerfs, and almost as much as her ult gave at max level before the nerfs. And that's only with 200 AP. 10 free AD and 70 more HP shield (still not counting ardent) per 100 AP.
So with 600 AP on Janna, you've got a 630 HP shield, and give your lucky ADC a free 100 AD as long as the shield is up. Though on average, I'm pretty sure you won't be at around 600 AP on Janna. However, with 300 AP, your shield gives a pre-nerf Vayne ult, a significant chunk of free HP, on as little as a 5.5 second CD depending on the runes.
Let that sink in for a moment. Pre-nerf max level Vayne ult damage buff every 6/5.5 seconds until the shield is popped, with just 300 AP and 40/45% CDR. For literally any champion it's slapped on. And it lasts for 5 seconds unless you pop it.
2600 crit item
1300 armor pen item 
2900 armor pen dmg item
1300 armor pen dmg item 
, along with boots, and now I guess 1 AP item. For the sake of argument, let's say
. And also for the sake of argument, we are ignoring runes.