Zombies are not made for combat. They are just bunkers for Necromancers. I've played them in a unit of 30 and horde formation with my supporting necromancer inside. Usually they just wander around apparently aimlessly, screening magic and shooting if needed. If my opponent wastes shots at them, that's only good. They just escort the necromancer around where his spells are most needed.
If my zombies end up in combat, I have made some mistakes in my movement plan and/or deployment. They work best as screens and cannon fodder and in a successful game they will block an essential charge, but only in the 5th or 6th turn, where my opponent doesn't have enough time to crumble the unit and charge another, more important unit.
If they are about to be charged, the Necromancer flees the bunker and tries to find safety in another unit close by. Also the broader front rank is good in the deployment phase, as I usually drop the horde in the center as my first drop, and then I can put the necro on either end of the rank. Never in the center of the rank. That way even his 6+ IoN bubble will assist the unit next to him and I can proceed to deploy my main force on either side of the zombies, depending on what my opponent drops.
In conclusion, I never, ever take zombies for their close combat value, as there is none to be had. I still consider them a valuable and useful unit for their enhanced ability to pop up and their low point cost.