Super Munchkin help

[D-heimer]

In a few games I've played with my friends we have encountered problems with the super munchkin cards and have solved most of them, but there is one we haven't been able to clarify.

My friend was a super munchkin, and his only class was a theif, so he would discard a card and try to steal from us, and if he got a roll of 3 or less, he wouldn't take anything, but he said that because he was a super munchkin and had none of the disadvantages he didn't have to lose any levels.
By the end of the game he had almost every single one of our items, and so it seemed unfair, but we couldn't decide whether it was fair or not.

Is this how the game works in this situation, or are the only disadvantages that super munchkin applies to the monster boosts against specific classes?