That they are rewarded so easily. As a mage, you have to constantly play your best to keep them both at bay - mind you, just to keep them at bay, because killing either becomes impossible if they're any decent. If you kill Zed, it doesn't matter! He'll just get a hexdrinker, rendering your entire kit insufficient!
You can get kills on them and still, the slightest mistake on your part will throw them right back in the game. If you, as a mage, make an early-level mistake and die, the lane is over for you.
It's just plainly stupid.