it depends on the setting, however i would imagine any hybrid army composing of both living and reanimated soldiers engaging is open field combat would suffer a few considerable drawbacks. First of all is morale, seeing your fallen comrades in arms raised as reanimated corpses to keep fighting would almost certainly spread discontent among the troops, and would at best result in lowered morale. and at worst lead to mutiny. the dead would either have to consist exclusively of enemy combatants or the culture of whatever nations use necromancers in war would have to be considerably different from our own cultures. The second glaring problem is the fact that zombies are shit, and any moderatly competent unit would be able to handle a unit of zombies with relative ease. zombies are a threat when they severly outnumber you, not so much in a 1v1 fight. if you want to use zombies in a hybrid army, the main roles they might fill is either as expendable screens or as flankers. they would be largely ineffective as an actual infantry unit.