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I understand you perfectly, especially regarding the mages. Unlike the Necropolis, they always lack everything (which puts a strain on their morale), but that's not the worst part. The worst is when you start playing with them on the same map after playing Inferno. When a hero of level n has an attack and defense of 2-3, it's infuriating! It's like playing as a neutral faction! Meanwhile, the Demon, unlike the Mage, has an attack that is 20 points higher + constant luck + skilled attack with all slots filled + hellfire... And yet, to say that the Mages have better magic... more diverse magic - yes!
Therefore, the units themselves are not weak for the Mages; in fact, quite the opposite (although I spoke out against them, now I repent, I have realized everything). They just look pale when the hero is a Mage. (in this case, even an Archdevil will look more impressive than a Titan - because its damage is doubled by the hero).
Mini-artifacts help, but not as much as, for example, a 50% bonus to the army through summoning in Inferno... and by the time you create decent mini-artifacts, you can lose a lot in development or not even live to see that moment...