Unit cost and quality.

I'd like to point out that with the first strike, the Nightmare's value increases sharply, as it is the strongest unit among the level 3 monsters in terms of health and damage.

I agree. The Nightmare is quite a tough unit and can easily defeat any Barbarian. Plus, it has some magical abilities. So, there is room for strategy. You can think about who to terrorize and who to give a good kick to. :)