I doubt anyone would waste a turn using a ballista on them if there's even one more priority target. And the ballista might even break by the time those priority targets are gone.

The point of the armored units is that they protect their neighbors from ranged attacks.

If the hero is going to buff everyone, then it wouldn't be a bad idea to take Adepts and use the dispel magic spell to remove mind control or berserk, allowing the hero to buff the creatures without interruption.

Dispel doesn't work on berserk. Mind control on the hero is not removed very often due to the level difference. And you only have one dispel spell (or more with magic secrets). It's not very reliable.

It seems like we're not talking about effectiveness compared to other factions, but about the fact that there are demons that play a chaotic role, and such demons exist (even if there aren't many).

No one is stopping you from leveling up a human in 4 schools and using spells. But the effectiveness of this is questionable.

1) The ability to upgrade Imps isn't just about draining the enemy's mana; you also need to use the fact that the mana goes to the hero/cave dwellers, and that also plays a role.
2) What guarantee is there that a counterattack will definitely be removed? Even though other Inferno creatures have high initiative, this doesn't guarantee it 100%, especially in the middle and at the end of the battle.

1. The alternate upgrade for cave dwellers is almost always better, and it doesn't require mana. The hero usually has a lot of mana anyway. So, the process of draining mana is more important than where the mana goes.
2. The hero often has the "Dispel" skill, which allows them to jump in without consequences. All demon upgrades reduce initiative, so someone will be able to remove the counterattack.

Can't agile units compensate for the damage difference with the damage they deal through "hit and run"?

So, without considering abilities, the damage of both upgrades is approximately the same. The upgrade completely ignores armor. Let's say the target has 40 armor, and the lizards have 20 attack. Completely ignoring armor will result in an approximate 200% increase in damage in this case. For the alternate upgrade, you need to hit 8 targets 8 times. But the enemy might have more defense.