>>280679930
The biggest problem with undead armies is that necromancers are overdone and often done in boring ways. They ignore the necromancy magic side of things and act as a knock off summoner that can only use undead. Where's the Frankenstein angle where you stitch together different corpses and then reanimate them into undead abominations? Anything that isn't necromancy will be far more interesting.
With summoning you have a wide range of options that let you party build and brings up more interesting angles to work with. This let's you bring in a lot more variety than a necromancer and undead army could ever bring.