Autoresolve on necro is bad, but that’s pretty much universal for necromancers in all startegy games. It’s hard to program simulated battles that takes in the odd intricacies of necromancy.
Undead can heal with a lot of various means though, I haven’t used the spell Cannibalize in like 20 weeks of game time. Unless you’re referring to a different spell?
Most of the units can learn Consume Corpse which is an easy skill to get end-of-fight heals, although I think it’s broken because it says adjacent corpse and has the action range of 1 but I can target any corpse on the field.
They can also level up and learn Melding Flesh for passive overworld heals. And there’s runes/souls and a level-up specifically for your spellcasters that can teach them Undead Healer, which gives a POWERFUL squad-wide auto-heal on the world map as well as a decent in-battle heal. And there’s also an enchantment that you can get that gives the same level of auto-heal on one unit.
But, of course, the best part of undead is their ridiculous access to lifesteal and how powerful it is. My main squad hasn’t taken a loss in a long time, even while warring against multiple mages at full power, because of lifesteal. Lifesteal on T2 acolytes, flayed ones, or skeleton riderse is decent. But when you can get lifesteal on a blood golem, he practically becomes immortal. Blood-charged is hilariously overpowered with the right strategy, which also gives a tiny heal to undead per turn.
If you can get Baron Meldec’s soul and put it on a vampire, you also have another near-invincible undead. Necromancers are meant to have a bunch of throw-away units at lower levels and carefully protected but powerful elite units at higher.
Also; of course necromancers use a ton of mana… that’s pretty standard across the genre. But they also need virtually zero gold, only to buy runes/items on occasion. Don’t invest into gold sources, invest into mana and you’ll be fine..