I've run Promise of Power in a couple campaigns now and I've been really impressed. I don't think you need to have any Curse payoffs in your deck to happily play it. It's just a good deal in its own right.

I think Promise of Power is considerably better than Unexpected Courage, especially on higher difficulty levels. Getting +4 to any test for any investigator at your location is premium--compare cards like Rise to the Occasion and Inquiring Mind, which (in my opinion) feature much larger downsides, while only giving +3! With a +4 bonus, even when playing on Expert, you have a real chance at passing a skill test in a weak stat. That's a nice impact for one card, and something like Unexpected Courage is simply not powerful enough to achieve this.

The Wild icons provide a welcome element of flexibility to Mystics who are otherwise focused only on Willpower. Scenarios often feature important non-Willpower tests, which can be awkward for hyper-specialized Mystics. This card is a great answer to that, and of course also has the flexibility to help allies cover for their own weaknesses.

Moreover, a single curse token in the deck is a very small cost. There's a very high chance that it gets discarded on a test that passes anyway (or, less probably, on a test that would have failed anyway.) I will happily take a large bonus to my skill test right now, when I know the test is important and I know the bonus is useful, in exchange for a smaller penalty later, which might come up on an unimportant test, on an important test that I committed a lot to and therefore can succeed on anyway, or simply not come up at all.

Finally, this is Practiced. That's very relevant because Practice Makes Perfect is a great card, significantly above the curve for a level 0 card and well worth building around. Together with Deduction and Perception, this makes a really nice package for investigators like Amanda Sharpe and Daisy Walker.

A promise is a bridge to the future built on faith.

Promises are easy to keep if you know what will happen.

Jacqueline Fine can give you three options, or a bad option with no chance of automatic failure.

With this promise, she can evade the cagiest enemies, punch the deadliest foes, or pass truly remarkable literary challenges.

If she’s recalling the future, those heights go from extraordinary to ludicrous.

Perhaps you’ll fail a future challenge because of a promise. But as you draw three tokens, more than any other mystic, failure comes on your terms.

This card is very strange. 4 icons is a lot. 4 icons is amazing. However, it happens to also be a Mystic card, which means the wilds are mostly going to act like pips. Adding a curse token to the bag is not that steep of a price in my opinion, especially if you're leaning into it with maybe one or two other cards that let you mitigate or benefit from curse tokens, though it remains to be seen what those payoffs are. Either way, 4 icons is a lot, and it can even be used to help evade if you don't have a good spell in play.

Realistically I see this as a staple for Amanda Sharpe. All of her stats being at 6 is amazing. Sure, she'll add up to 3 curses to the bag that turn, but that might actually be a good thing, again depending on the payoffs for curse tokens. Patrice Hathaway also likes it, as she's already happy to commit all of her cards in a turn, and this is just a very easy one to throw down.