Routing a unit is no easy task, especially if they got high numbers or high nerve score. It requires prepunishing with ranged attacks and spells and either a storng charge with a superior unit ( for example heavy cavalary) or coordinated attacks from flanks and rear.
Otherwise they are not effected or just wavered for a turn.
Undead units do not waver but can only be routed. But it is hard.
For example zombie legion ( 60 models, 10x6) has -/28 nerve score. (-) means it can not be wavered since zombies have no brains

In order to get 28 on the dice you have to deal 21 wounds on them to have a approximately %50 chance to rout them. Since they have life leech rule, they are healed 1 wound (2 if they got a 10 points (very cheap) upgrade). and you can always heal (ressurect) them with your necromancers and vampires.
Imo, Undead army needs only raise dead spell in order to be played just exactly as vampire counts.