Necropolis and Demon.

It depends on who you ask. If you use your troops effectively, the Demon Lord can have the upper hand. Also, factors like fighting neutrals and other heroes should not be overlooked. Considering the fragility of the Necromancer's army, his losses will only be compensated by reinforcements if he engages in intense battles. In that case, the Demons have a better chance. By using the portal ability, they minimize their own losses.

Could you describe in more detail how to use troops effectively?
No matter how fragile the Necromancer's troops are, they will suffer fewer losses in battles against neutrals than any other army... even in intense battles, you usually only lose a few units... especially when a couple of ghosts appear. And the troop increase from necromancy is almost equal to the troop increase in the castle during the first two weeks. Later, when you can build 1-6 monsters in the castle on the 3rd week, necromancy will bring in relatively fewer troops than the castle will produce on the 4th week, but including the first month, the Necromancer usually has twice as many troops as his opponent. As a result, he usually levels up faster and gets all the good stuff on the map earlier.
From the start, the Necromancer can handle ranged neutrals and difficult neutrals like fairies quite well. What will the Inferno hero do with only imps and demons to summon? In the first round of each such battle, he will lose troops to ranged attacks or fairies, and in the following weeks, it will be almost impossible to defeat, for example, a group of archmages or druids without losses. But the Necromancer can. And I'm saying all this without considering overpowered heroes and skills. For example, if you take Kaspar from the start, the Demon will simply not have a chance to survive until the 3rd or 4th week... even with a starting Deleb, although she is weaker than any other "summoner"!
P.S. We can play an online game and check... are you ready?