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Djinn are actually still worth something; the rest are a waste of money.
Well, assuming grandmothers aren't being sold. Although, compared to Heroes 1, djinn have been significantly nerfed. Medusas in the Seven Lakes scenario helped me seize the wizard's castle around day 8, and it seems I was already fighting giants.
Around a hundred grandmothers can easily be defeated by four red birds from the Warlock's castle, led by a decent hero (an advanced Knight or Barbarian) without any magic.