What is the source of the Mages’ power?

What do Hafiz and gremlins have to do with it? MjTh says that three archers will result in a unicorn bow, which will give a slight advantage over other factions. What advantage, if archmages shoot at maximum range with or without a bow? That is, it will help mages just like everyone else: it will make two units shoot at the entire battlefield.
And by the way, the unicorn bow is available to any faction. Dugala's crossbowmen or Haggeesh's Kents with a bow will defeat Hafiz's gremlins, and Ossir won't even need a bow for that.