The mirror mode is actually balanced; it's just that, over the years of playing Heroes, some people have become so accustomed to blaming the random number generator for their losses that they automatically look for the generator's flaws in mirror mode as well.
I think that even when Wizard makes the taverns identical, there will still be some unique individuals who will claim that the balance is still off.
I remember Alex once explaining his view of the generator's role in the outcome of the game (not verbatim): "If I lose, I analyze the game and think - did I play perfectly? If not, then the reason for the loss is in myself."
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