It has great range though (900-ish if you activate it at it's max range of 650), and instantaneous. I almost never have to use it defensively because it's impossible for them to get close without playing right into my hands. If one person gets too close, they're vaporized and the fight is a 5v4. If more than one person gets too close, then I've stunlocked a large group of people and the teamfight is practically over.
And tenacity is a 35% reduc in cc duration. That's only around a .8 sec duration reduction, and not every person gets tenacity. An aoe 1.7 sec stun is still enough to win the fight, as it gives you more than enough of a time window to vaporize somebody.
It's probably just his relative vulnerability to getting camped and his weak early game, but he just needs a little babysitting and he starts rolling by 5, wrecking by 6.