I think that depends on your choice or preference. However, as a quick-and-dirty rule, my recommendation would be to demonstrate the MC Unique Ability once per Act. As to whether it achieves the Goal, I think each time it appears, it should suggest to the audience that it has the power to bring about Success, even if it doesn't immediately do that. You might even have a scene where the characters' progress is thwarted because of this trait: for example, a character whose Unique Ability is their kindness and self-sacrifice might unwittingly fall into the bad guy's trap because they try to save a helpless bystander. However, this scene would still be framed in such a way that suggests that kindness will always overcome in the long run.
That's my take, anyway. I could conceivably imagine a story where a character's weakness turns out to be their true strength, and even the audience is fooled for most of the story. (Sadness is the IC in Inside Out, but her powers of empathy and vulnerability might follow this pattern.)