Secondary skills in Heroes 3.

I'll share my opinion:

Luck is a weak skill; it works when units have fixed damage or double attack (Necropolis Knights, Marksmen).

But if luck triggers for Storms or Dogs, they deal half the maximum damage. So, I'd prefer any school of magic over them.

Incorrect definition of the Luck skill.
If Luck triggers, it adds the unit's damage again.
For example, an Angel attacks a Demon, dealing 115 damage. If Luck triggers, it will be 165.

So, Luck is very beneficial if the unit has low attack damage.
Your units deal little damage, but with Luck, their 100% pure damage is added, which can increase the damage by 2-4 times (since the damage is already greatly reduced due to the attack/defense difference).