To me it depends on the game. In support of this I'd like to draw a comparison between two old favourite zombie titles of mine.
First up, Left 4 Dead. In what can best be described as a gaming tribute to co-operative play and the 28 days later legacy you can move fast, you can fire your guns fast, you can store alot of ammo, you can carry health kits, molotovs, pipebombs and of course...peelz. Along with this you have three other chums, you can take a fair bit of damage and you have short and sweet sections of the campaign before stopping for a breather to gather your senses and supplies in saferooms. Now to counter all these advantages the zombies, while having their numbers whittled away like gerbils in speed bags full of broken glass, are extremely fast and travel in big hordes. As well as this are various special zombies that have the potential to fuck whole teams over with the ability to do one hit kos, incapacitate, stun, draw zombies towards you and get the wrong idea of a c&%$@ingis.
Second up is Land of the dead: road to fiddlers green. It may have been a movie tie-in but being the big fan of zombies that I am I enjoyed it. Basically you can't move faster than a snail on anaesthetic, you can take a realistic (For a game with flesh-devouring monsters) amount of damage, you're reloading and indeed firing skills aren't up to scratch and you can carry very little ammo. But because of these disadvantages the zombies are slow, shambling and stupid.
If we switched protagonists around in these games the difficulty curve would be FUBAR.