I think people are missing the point of Opportunist, and this is causing them to analyze its value improperly. Note that you are note forced to use Opportunist on every skill test. You get to pick and choose when you want to use it.

As its name implies, Opportunist is a card you use in a situation that is already to your advantage. It is the card you use to push your +4 to a +5 to take out that small additional chance you might lose.

To evaluate it, compare it to Unexpected Courage. Unexpected Courage can be used one time to increase your ability score by 2. An increase by two will typically improve your success rate by around 20-25%. Fair enough. Let's compare that to Opportunist.

Opportunist, if you use it in the way I suggest (turning +4s to +5s) will increase your success rate by 7% on every skill test you try it on until you fail. The likelihood of failure is around 7% (autofail), so the longterm value (based on mathematics of infinite geometric sequences) is 100% added over the course of several skill tests.

Of course, you are not going to get this full value because the game is only finitely long, but you should easily get 3-5 uses of the card during the course of play, making it at least on par with Unexpected Courage.