JohnSnow
Hero
The cost is that typically in DCC, Deed dice add to damage as well as attack rolls. If you choose to perform a Mighty Deed, it might not happen (2/d chance), but you give up the extra damage regardless.So the cost is, in theory, if you roll low enough on the die, not only does the special "thing" fail to trigger, but the attack/ check/ save or whatever could fail as well?
I suppose a lot would depend on what stunts you could do, what the number on the you have to roll to succeed is, etc.
So it’s not a high cost, but it’s not no cost.