Probably this is a stupid question, and maybe it's one that has already been asked and answered. I didn't find it though, and even if we are now playing for some time this situation happened last game on a dramatic moment. So I was in doubt.

If an attack action failed, but the roll generated some boons, and the Action Card has extra damage boon effects, do you apply that extra damage alone, without adding Strenght and the weapon DR, or do you consider this boon effects as unable to apply?

Consider Reckless Cleave being used with a Reckless Stance. The attack failed, but generated a lot of boons and a Sigmar's Comet. That card has a 1 Boon Effect that says: "You may add 2 recharge tokens to this card. If you do, your attack deals extra damage equal to your Strength." The Sigmar's Comet Effect is: "+2 critical" . So that attack could have missed but something occured by luck or fate or Sigmar's Grace and the target of the attack ended suffering Str Damage, 2 of them being Crits?

I really think that could be a possibility. Thanks for your patience in answering!

On a failed check, you never apply effects that modify the result of the success effect. If an attack misses you won't do +1 damage, because you missed entirely. You could get a free manoeuvre on a failed check though - or some other effect that isn't modifying the main effect.

As Gallows said effects that modify the damage of the attack don't apply if the attack failed. However other effects on the card may trigger like a free maneuver for example. Also the universal rules apply that for every 2 boons you can remove one stress/fatigue (remembering that a comet can count as a boon). Additionally I try to encourage my players to use boons/comets in other ways not on the card.

One example is a player suggesting that even though their attack missed it still put their opponent off balance so I gave his opponent an extra misfortune die on his next attack.

The key is to not allow players to do the same thing over and over again that it just becomes a staple. Only allow them to do extra stuff like that if they are being descriptive, original and it fits the actions at hand. Also when I say original I don't necessarily mean it's got to be 100% brand new. But just varied up so that it doesn't feel stale. Also you may want to limit those kinds of effects to once per opponent or per combat or per scene.

The bottom line is to encourage creativity, encourage players getting in on the action that exists beyond their cards and to create a flowing events that exist between I swing I hit or I swing and I miss.

Thank you two very much, it was very helpful indeed!