Isn't the entire point of every undead faction (in any game) to be that - the dead soldiers are just better - this is just a logical extension of any other army. Since we can do it; why are you not doing it? If we both had dead solider's you guys would have a better chance.
If undead wasn't inherently better in some way - they'd have no argument and you wouldn't get undead factions. No one's going to buy the logic "Hey I killed all my guys and then spent years turning them into the walking dead - now they suck more and gained nothing from it! Yeah, I'm an evil genius!"
One assumes that it's factored into the costs of units that get it - and that would go along with the fluff logic of them being an investment (You need to get well trained warriors - then kill them, and magically reanimate them to get well trained dead warriors)