First consider the general traits of the undead. Let's do DnD 3.5, for example. If you just look at the list, you'll see that they're MUCH better than living humans, for example. Even the weakest skeletons don't need to breathe, eat, and sleep. They never become tired, and can't be routed (well, maybe intelligent undead can be, if they think they lost the battle). Basically, an undead army will always be superior to a living army of a comparable strength just by the virtue of its resistances and immunities, if we're not talking elite troops.
Then consider individual undead unit types. Skeletons have resist 5/blunt, zombies have 5/slashing. 5 damage in DnD is like a sword swing or an arrow hit by a common soldier, so they're both very resistant to arrows. So even the weakest undead creatures need to be dealt with only with specific tools or magic.
Next consider the elite undead, that can't be hit by non-magic weapons at all, or have fear auras, or have poisonous bite, or can turn humans into ghouls. Even a few of those creatures used as shock troops will decimate regular living army formation if there are no counters to them.
Finally, consider what magic and technology can do to augment the undead strengths and cover their weaknesses. Suicide bombing with skeletons full of bottles of alchemical fire, zombie bats dropping poisonous gas bombs right in the middle of the ongoing battle, necromancers casting cloudkill that moves along with the advancing skeletons, and so on.
TL;DR: whatever you do with undead, if it's not throwing them blindly into the fray against elite troops, it's going to be pretty spooky to the living armies. And if you have access to magic or elite troops, it becomes downright horrifying. Dropping infected zombies into the city severs, vampire assassins, chemical and germ warfare, death spells.