"War by the rules"

I don't understand what's so good about Dwarves here? Or am I missing something? And why are Mages rarely used?

With large armies and the ability to develop for 3+ weeks, Dwarves are actually quite good because runes are powerful. The machines are what allow 99% of players to successfully start a game for any castle, but the main drawback is their logistics.
Mages are an unstable race, but very dangerous due to their mini-arts, which can adjust the potential of the army in any direction, plus the universal spell and the artifact shop, and of course, Gremlins against any artillery and the overpowered Hafiz (as an additional hero, especially if you rotate him a couple of times during the game).