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In King's Bounty, different types of damage are often combined in a single attack. But some units (like the bone dragon, for example) deal exclusively poison damage.
By the way, here's why teleportation is associated with demons? I can imagine it even as a walking ability :). Moreover, regarding the limitation, I'm expressing my opinion, especially since this aspect was balanced quite well in Heroes 5. And black dragons should fly further because… they are the strongest unit, after all! And they don't have any special abilities!
Different units need unique features, but teleportation is just an animation. In the game, teleportation is no different from flight. So, why make this animation better than the original?