Hey Ozle,
Yeah, 'unbalanced' would be generous. For the humans, it is random chaos piled on top of more random chaos, and yet your objective usually centers on finding a single needle in the chaotic haystack. The zombie players' cards, however, are almost always useful; they are actually quite well designed, thematic and devious. Humans, not so much. I figure the only way to fix the game would be a pregame deck set-up of that human haystack.
For example, a common objective is to get ~2 gas can cards and 1 car key card from the deck, start the truck and get out of dodge. The deck has (I'm guessing) 100 cards in it, and each player could draw around 2 cards per turn, maximum--assuming they weren't running away from zombies or firing their peashooters. Since the zombies only take ~2 turns to close on the humans in any given scenario, that leaves you drawing roughly 10 cards before you absolutely must start performing actions other than searching the deck. And the zombie cards are all the while messing you up. Estimated %10 chance of finding what you need before the slaughter begins. I don't remember the dice combat values, but I think the zambies have the advantage (on top of weight of numbers, they certainly do).
So, if you want to feel like you're a stock B-Movie face-chomped farmboy, or else gnaw on your buddies' delectable brains, this might do the trick. But you'll very rarely be Ash, and the game mechanics are often very frustrating.
What else are you looking for in a game? You mentioned casual friendly. Is theme a big factor?