His Q hitbox is broken (I mean ltierally not functional, as it is not as big as the graphic, but bigger), his Q has less cooldown than his passive which makes him able to stun you without being in auto attack range and just using his Q. This is going against the basic design he got when being released. His percentage damage is way too high, compared to his tankiness, which is even higher due to his low E cooldown and huge refund he gets when not using it. Even when he builds not a single tank item he is just like a normal tank, but at the same time an adc with insane croud control and burst damage output. His ult is actually not too strong but since it teleports an unkillable ad carry tank bruiser in your backline (plus any teammate), it is now.
Also, his auto attacks scale on health. I don't know any champ whichs auto attacks permanently scale on a base stat that is extremely powerful on the champ anyways.
I do play Tahm myself, so don't say I am just pissed because I lose against him. He is way too strong. I also see it when playing him myself. Just nerf him already, reduce his percentage damage, revert his Q cooldown to the earlier, higher cooldown or reduce the passive duration. Increase the E cooldown.
And for the love of god. Don't give a tank auto attacks that increase in damage when he builds tank. That's not smart.