The real problem with Zombis is that to get them work (if your opponent is not giving you opportunities) you have to play them from your hand, costing 3 cards total, and you still have 33% chance of failure. Even when you activate it, they can turn into magic fodder for your enemy unless you have a higher priority target in sight, like a champion. And they are not reliable early game.
Those are too many 'buts' to really like the unit or make win you games reliably.
PS: almost every faction is good at playing all of their champions