He has too much damage for a utility tank front liner.
Passive grants : 20 - 71 (based on level) health and 3.5 - 7.75 (based on level) mana, That is insane.
Q Has 100% ap scaling has a 60% slow at all levels and knocks you up..
W silences in an aoe still scales decently has a huge hit box so basically any melee champion cant trade with cho gath at all and even if they do he has his passive so he just out sustains.
E Probably the only non oppressive ability he has in his kit .
R Is fine besides the ridiculous damage to champions.
It would be different if he was more focused around being tanky and being a cc frontliner which he is played as but he has way to much dmg for what he has in his kit.
yes he is immobile but you have understand he has a knock up a 60%slow and a giant aoe silence and he builds tanky so the chances of you killing him are slim.