You seem to not understand his weaknesses... He has a Skillshot for his Q, a fast one at that, but a skillshot nonetheless. It is easy to dodge them, just don't be shaggy and Scooby running away style. His Devouer is probably one of the most easily mistakable moves if you have another champion on you. He goes after health for his ult, which makes him vulnerable to damage, his E is easy, just poke him down and wait for his shield to go off. His ult active, you need to have Map awareness. He is not only a support, and he would have to level up Devouer to the 5th level, only occurring at lvl 14 usually, to get that 32% max damage. For his scaling on his q, have you ever seen a Tahm Kench build pure AP? He is suppose to deal damage to get rid of assasains on his backline, scare them away. He is meant to both peel and engage, catch people out, and most of all, protect the ADC. Any decent Tahm usually saved their devouer for saving their teammates from a Cait or Jinx ult, and to be able to waste any enemy's dashes or jumps.
TL;DR - He isn't broken, people rarely play him, which means that people rarely know how to play against him. Watch for his gameplay mistakes, capitalize on them, that's how you win against Tahm.