I think the more fundamental problem (Well, maybe problems a bad word) with them is the battlefield role they play. Damage is always the first priority, even in a mission based game. And those units are the damage units. They're the frontline fighters and they're supposed to be. Other models fill various support roles and have abilities that support ones army and pay points for this. Models like the zombies and marshals (And some others) just pay for flat damage. Every game has the front line troops that make the core of the army, they just happen to be the core for those two armies (And lets be fair. Witchling stalkers, the Ortegas, Crooked men, Terror Tots, and a number of other units are comparably good.).
In smaller games they'll make up the majority of the list but the larger the game the more room there is for support (like in all games. Frankly I like the game more in the 35-40 range. 45 is good too if you increase the hand size by 1). And personally I think most units in the game are at least comparably powerful. For most units I think the power level difference is relatively small, especially compared to other games. You know, maybe .2-.3 SS here and there.