It is important to maintain distance so that the Necromancer can only hit one stack of Black Imps with a single spell.

If the Necromancer is using Chain Lightning, and he keeps casting it on a single stack of Imps, that gives the Demon a huge advantage.

Speaking of Chain Lightning, there's also Lightning Bolt and Meteor Shower. The number of spells needed depends on the size of the stack. 200 Imps can be defeated with two spells (with an amulet), and even after one spell, the Imps won't be able to generate mana as quickly. But when the stack is 500, Chaos magic becomes ineffective. And the Necromancer won't bother developing Chaos magic on a large map; it's better to go for Howl of Terror (which, by the way, makes any Imps useless). By the way, Imps are usually a priority target for neutrals and are eliminated fairly quickly if the hero isn't Delaeb.

After summoning, there's only a half-turn cooldown.

Yes, I forgot to factor that in... and there's also "Quick Portal Opening."

At the same time, it's not that easy to reach the Imps with an army.

If they are standing separately to avoid the Cold Ring, even a dragon can get through. Especially vampires, who can heal well from Imps.

The point against Spiritual Link is that the Imps will drain all the new mana.

Hold on. Let's define the army sizes. If there are 1000 Imps (without summoned ones), the Necromancer needs to deal a little more than 9000 damage per turn to a unit with Spiritual Link to replenish 125 mana per turn, otherwise, it will lose all its mana in two turns. But the trick is that the Necromancer can deal only 1500 damage to the linked stack before his turn, and the mana gained will be enough for one Dark spell, and the Necromancer won't be able to cast more spells in his turn. And let the Imps drain everything to the end; before the next turn, the Necromancer can replenish the mana to the required level for the spell again. On the other hand, since Chaos magic doesn't play any role with such armies, the Imps can be neutralized with Archliches or Bone Archers. I think it will be with one shot, since the Necromancer's army should be many times larger under other conditions.

If there are about 200 Imps, they will drain the Necromancer's mana for at least five turns, without taking Spiritual Link into account. In that time, something can be done to them, and Chaos magic still works.

In general, it all depends on the Necromancer's desire. He definitely has a counter to Imps, and often they will be more useful in attack.

Not that I completely reject such a tactic (Spiritual Link may not even work for the Necromancer), but to always take Imps because of this potential opportunity to put pressure on the enemy...