Boris K
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Humans are strong due to training. And for light or dark magic, powerful spells aren't necessary.
The weak attack and defense of mages are compensated by mini-artifacts and the hero's spells. I don't really like playing with them because it takes a lot of time to craft mini-artifacts. And this annoys my HotS partners.
Dwarves are strong due to runes.
It's difficult to play with demons at the very beginning. But later, they become quite competitive.