What kind of magic do you like the most?

If that's the case, then there's nothing surprising about it; darkness, even with Sandro's cloak, is not very effective against Necromancers. If you're playing Necromancer against Necromancer, I would probably choose Summoning and Chaos/Light.

Okay. But the fact that darkness isn't always effective speaks against it ;) While any faction with Summoning in its guild can calmly use it in absolutely all situations.

Not at all. And if you just compare, it might. There are many of these spells.

The point is that if you consider the neutral school, it's out of competition because it actually makes up the largest part of the game mechanics. Almost all hero skills (not just active ones, but passive ones too!), a huge number of creature abilities – all of this are spells from the neutral school.