I'm thinking, wouldnt an undead be, by definition of not having a brain or even being alive, be the perfect suicide assasin/canon fodder. But, you would need a method to keep the enemy side from recycling the boys a second time. If you dont, its like playing pingpong with a massive undead horde. A sport I'm sure many necromancers would enjoy playing. Also, hig lv clerics and paladins wold be able to, literally bless the rain, killing undead left and right.
On a side note, would the ghost of a soldier still want to fight? If so, the necromancer just got the posivility of more complex tactics and strategies, thanks to these ethereal aides.
I've read this saga on the napoleonic wars, but with dragons called "Temeraire" by Naomi Novik. It depicts an amaicing array of ways in wich diferent cultures interact with dragons. India, Turkey, England and specially China have diferent ways to interact with the lizards and the book also goes into detaio regarding the tactics andadvantages that dragons bring to the table.
But vampires would be the kings of the night if dragons are the kings of the day. The relative implications of mass vampirism are much less concerning than winning the war after all. Also, you can make vampire-spawn by the docen and turn the promising ones into full fledged vampires, as vapire-spawn are under the absolute control of their master, they cant revolt or refuse to charge. This also opens the door to vamping any prisioners you get to extract information from them.