Main reason is :
His very very high damage output. The highest one in the game. He can easily slap 3 of your tanks into nothing, all at once.
Yes, I am dead serious, if you fail to kill him first, or thought "Nah, I'll leave this Zerk later", then he activates his skill, OHKO'd one of your unit, you decided to neglect him, then he gets his turn again, and boom. RIP whatever you have in front of him. I have experienced this once. Almost had a heart attack.
Well, he is a bit squishy, but that can be taken care of later.