A situation is imbalanced if it's not equitable or fair. An imbalanced workplace might pay men more than women for doing the same work.
Something that's imbalanced is off-kilter or out of whack. It's out of balance, but not in quite the same way that the adjective unbalanced implies. When you describe something as imbalanced, you're likely talking about a rule, a law, or a procedure, while you might call a shaky wheelbarrow unbalanced. Things that are imbalanced are out of proportion — power itself is something that's frequently imbalanced.