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Why wouldn't a fairy be a good substitute for a demon? It flies just as well. Plus, it's ranged and faster.
Под холодную Луной порублю попсу косой (с) Decimacia
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No, not at all... in principle, only if you have an army from the Castle of Death, then the Fairy will have low morale (and the Inferno branch, it seems, too).
It would be better to try and determine the most useful monster under any circumstances.
One that gathers resources itself and is suitable for any army. :)
I think everyone needs luck.
A Fairy is better than a Demon. It also flies, it also has a long range, and it attacks with a retaliatory strike. Although it won't be able to steal mana from neutral creatures. But that's a specific case. In general, Demons are also good and that's all there is to it. And of course, they are more useful than Gnomes:)

What about pirates?
What's wrong with them? They aren't obsessed with you.

And pirates existed before, they have already been described, all their advantages and disadvantages have been mentioned.