Lets talk about this card, with dummy probabilities. Because numbers are tricky and I think I can explain it simply.
First off, this is a "cantrip", it's a cheap/free card that draws another card. Why is that very good? Most arkham decks have a few cards that you cannot go without, your most important cards, for example a weapon for a fighting , spells. Having a cantrip like Tempt Fate means that when youre sifting through your 30 card deck to try and find those key cards, you're actually sifting through a 28 card deck.
But, it is'nt "free", it comes with the bonus / penalty of and tokens.
Without going into exactly how, / tokens are inversely more powerful depending on how many tokens on a chaos bag net you success or failure. When the chance of success is low, is more powerful, the inverse is true for .
I.E. If you're doing a lot of tests that you know you'll win just barely (most of the time you'll be avoiding to do tests if your chance to beat it are negative!) then curses really hurt you, similarly if you do a lot of tests you know you'll just barely loose, then the blesses will help more then the curses hurt. This effectively means that Tempt Fate hurts consistent characters with good baseline stats, but helps risky characters with a glaring weakspot (Like many folks who find themselves doing tests from low starting points.) Incidentally, this means that strategies can be very helpful to middling characters like Roland Banks, who find themselves throwing repeated Investigate checks at shroud 2 or 3 locations.
To explain it more detailed: You can calculate the most common token draw from a bag pretty easily, many // tokens are net -1's or -2's on standard, and on standard -1's and -2's are common. If you test such a chaos bag with a baseline of +2, and then add blesses, your chance at success increases only by the amount of tokens in the bag that a bless might help with (a -3 or -4), but because those are less common than the opposite, the blesses help less then the negative effect had by a similar number of curses, because there are more tokens that a curse can turn into failure, than there are tokens that a bless can turn into success.
For example: If you have five -0's in a bag, and three -2's (and nothing else), and you're testing from a baseline of +1 over the difficulty, then if you add a bless into the bag, there are 3 tokens where bless + token replaces failure with success, if you added a curse instead, there are 5 tokens where curse + token replaces success with failure. I.E, in this case, adding a curse hurts more then the help gained by adding a bless.