Kennen is just a toxic champ
Champs with built-in stuns on their passive are always obnoxious pieces of shit, but you threw all fun and counterplay in the trash when you gave him his ult.
Here's the Kennen vs. Gangplank battle that went on.
I got Merc Treads, and Phantom Dancer, and all that important stuff. Kennen only had a peasant's 250 AP.
Kennen had one stack charged on his auto, and used it on me.
"Ow," I said.
Then he immediately storms up to me with his 900 movement speed and easy 
dash and just started to ult. He didn't even Q or W me; he just ulted.
So he was able to land that longer-than-needed stun on me rather instantly, but I reacted quickly and ate an orange to escape.
BUT WAIT.
I'M ALREADY AT 40% HEALTH FROM THAT ALONE.
HE STILL HAS 900 MOVEMENT SPEED.
HE STILL HAS HALF HIS ABILITIES.
HIS ULT IS STILL GOING, AND DEALING INCREASED DAMAGE!
So that's Kennen. Massive damage, massive mobility, and brainless cc. He can 100-0 champs with his base stats alone, which makes it all the worse when you see a 3000-health tank-Kennen chunking everyone's health in an unfair, uncounterable manner.
takes a lot of skill hes so squishy and you just zoom in at over 9000 movespeed and press R and stun the entire enemy team without aiming anything and...
, who will adore flexible builds to shut down snowball aggressive champions with EZ.
then panick rushing for your damage items, most people keep forgetting a thing called cheap item damage shut downs. I've literally done the same against a lethality Jhin just a few hours ago as 
with a 
team mate and the enemy support was 
, granted Alistar is a perfect counter on disrupting Jhin and Thresh on grabbing a ADC and cc and damage harassing but soon as Alistar does a WQ then they are kinda ruined on that op.
can screw over his first R stun so you can quickly CC or damage blast him and benefit from that lifesteal and MR to blunt his damage nuke.
or 
in the first back to capitalize early on damage countering and make use of outscaling and turning a trade into a long one that most snowball champions can't aim for on high mana/energy costs or builds that will grant them little sustain or beef in the early game. Until the snowball meta is gone, Lethality ADCs, aggressive top laners and yolo diver junglers are going to be pretty rampant so best to play turtle and force them to starve while they wrecklessly slap at your turtle shell and then the power you built up thru a Bide will let you smash their faces in.