Veigar isn't an easy champ to play, for whoever listed him there. Landing the stun instantly will take a lot of time to master, specially when you want to stun more than a single guy.
AND, as an interesting fact: Thornmail reflected damage is calculated BEFORE armor mitigation. That means that if you have 50% reduction (100 armor) and tryn original hit is for 1000k, even though he'll hit you for 500, he'll get 300 dealed at himself (Supossing he has 0 magic resist) Another interesting fact is that Frozen heart is more effective against a fast auto attacker like tryn or yi, and it helps your whole team.
Last thing: Kog maw is fucking awesome, He tears through tanks and carries alike, and they can't even completely itemize against you. He's amazing, you just have to build correctly.