Dark energy

At the start, Necros only have Vlad and Kaspar...

Really? Someone like Zoltan easily reaches level 12-13, or even 15, within 1 week in the Outcast Lands, and they can quite comfortably take on the Dragon Utopia by day 8 (sometimes even day 6).

And for TE there's Waishan, Kifra, Letos, and even a Dark Mystic simply learns the second Chaos spell and goes to kill...

And why can't Necros have damage-dealing spells? Their situation with mana is actually better. At the start, Plague really burns everything down; Matyuk from the Dragon Utopia will kill a Warlock with one cast in the 2nd week—by the way, there's lightning from Druids and the guild is built as well.

But Necros still won't raise much at first...

Yeah, but even 200 banal Archers with Resistance (specifically in the 2nd week) is 1400 HP + raising dead. What are you going to break through them with?
When you effectively get about 580-600 TE for the first 8-9 days, that's a very serious addition to the army.