What kind of magic do you like the most?

But the fact that Darkness isn't always effective speaks against it.

Darkness loses some of its effectiveness only when playing against Necropolis due to the undead's immunity to mind control spells, but mass slow/weakness/debuff/disintegration beam are also good;). (Perhaps I was a bit hasty about "ineffectiveness"). With very large armies, Darkness can even be used against Necropolis, but in combination with other magic.
But if Darkness has one drawback, Summoning has many more:
1. The larger the army, the less likely it is that a Phoenix/Elemental will tip the scales in your favor.
2. If the hero has the spell "Deadly Cold," and there are Messengers or Pit Lords in the army, summoning a Phoenix is highly discouraged:D
3. In terms of mana cost, it is the most expensive school of magic.
4. There is such a ring of Banishment, it is inexpensive, it appears quite often in the shop, it may even be given at the start, and if your opponent has it, it is better not to develop Summoning).
You can come up with more, but it is obvious that Summoning has more drawbacks than Darkness.

While any Castle with Summoning in its guild can calmly use it in absolutely all situations.

Well, the choice of a Necromancer to have only Summoning seems like a rather dubious decision to me.

The point is that if you consider the neutral school, it is completely out of competition, as it actually makes up the largest part of the game mechanics. Almost all hero skills (not only active, but also passive!), a huge number of creature abilities - these are all spells from the neutral school.

Exactly, everything that has nothing to do with Chaos, Summoning, Set, and Darkness can be attributed to the so-called "neutral school," so it is wrong to consider it magic.