If an opponent plays Unchecked Power and I play Mystic Reversal in response, can I choose not to use Unchecked Power's effect (exhausting units and dealing 12 damage)?

Ruling: No, you cannot choose not to use the effect. When you gain control of the trigger with Mystic Reversal, it is already on the stack and must resolve as written.

Nuances:

• Unchecked Power says "all friendly units" without "up to" or "may" language, making the effect mandatory
• You would need optional language like "up to" or "may" in the card text to be able to decline using the effect