Jacqueline, while less specialized than other Mystics, can be quite a powerful investigator who's ability interacts in strange and beautiful ways with so many of the mystic cards we know and love. I also appreciate her 6 Health and 9 Sanity which makes her quite a bit tankier than one might first expect, especially once she's put on her nice fancy Robes of Endless Night.
5 Willpower - What you really want in a mystic. Such a high starting value in your primary stat makes you so much less dependent committing cards when facing encounter cards and casting your spells.
3 Intellect - Good but not great clue finding. Throw in a few flashlights and/or perceptions and Jacqueline can help gather a clue or two while saving her arcane slots for fighting spells if you decide to go that direction.
2 Combat - Don't expect to win a fight without a shriveling or azure flame ready to go.
2 Speed - Also don't expect to go passing encounter speed test or evasions anytime soon. This is what mystics bring deny existence and ward of protection for.
Overall I really like her stat distribution. Good support clue-finding and excellent willpower for all the normal mystic shenanigans. Her biggest detriment is the inability to deal with monsters without any assets or events.
I'd categorize her ability as having separate purposes that interact with either her own cards, or her fellow investigators.
With her own cards, her ability makes token flipping cards much more reliable. Improve the resource gain on Voice of Ra, proc cards that want tokens to be flipped like hypnotic gaze, or dodge self-damaging effects on cards like storm of spirits or shriveling. Additionally it becomes that much easier to gain additional cards through prescient and crystal pendulum. You can include cards like Olive, Premonition, or Scrying Mirror to support the token flipping reliability but I've found Jacqueline's ability was strong enough that it didn't require additional support.
When using her ability in a group, look for where success is needed and aim to end (or start) your turn there since you have to be on the same location as the investigator flipping the tokens. When assisting friends there are two instances in which i found her ability worked best, tests with a narrow chance of success (starting at +0 or +1) or tests you can't afford to get the autofail. For the former can increase the odds of finding a 0, +1, or Elder Sign and has surprisingly often passed tests that had no right to pass (note* I haven't done a full mathematical analysis on her token flipping odds, although I would love to and might in the future). For the latter, there are some tests that you JUST. CAN'T. FAIL. With Jacqueline you can more reliably dump all your cards into this important test knowing that even if you do flip the autofail you can cancel it. However this does create a new range of negative tokens in the bag to consider, for example flipping the autofail, the -5 and the -3 means the test is now a -8. There is an excellent analysis of this for the Olive McBride card and I suggest you read more up on it there, however anything that lets you guarantee dodge the autofail token is worth noting.
Her ability can be summarized by the word Reliable. Seeing the future is never a guarantee. But once per round, on one test or token flip, you can breath a little easier.
UNIQUE CARD and WEAKNESS:
Arbiter of Fates is cool, simple, doesn't take up an asset slot, and simply doubles her ability usage every turn. Great. Refer above to why her ability is nice to have in your pocket, now you have it twice and you don't have to feel so bad about using it to dodge a nasty encounter card. Pretty unnecessary for solo as you will be using your actions to play cards or move around more often but can be nice if you have the time and resources to slap it down.
Dark Future is also cool, simple, and just sucks to have out. Quashes all the benefit of Jacqueline's ability minus niche cases like Hypnotic Gaze or Voice of Ra where you want to draw tokens. It even spitefully turns off your ability to cancel the elder sign for cards. However, since you now won't be using your ability on tests, you now get to use it on this jerk of a card at the end of each of your turns in hopes the helpful blue token will appear and save you. Her weakness has both an extremely notable upside and downside. On one hand it does not require any action to deal with, it will simply go away on its own. On the otherhand... IF it ever goes away. Some missions you will end every turn, flip your 5 (or 7) tokens, and audibly sigh as you realize there's nothing else you can do to help speed the process along as you remain ability-less another round.
SHORT NOTE ON DECK BUILDING:
Although Jacqueline has a very limited card selection range, the way she interacts with mystic cards makes many more card options viable. Voice of Ra and Crystal Pendulum I would consider core to her, as they provide the resources and cards and Willpower boost that fuel her primary actions. If your deck utilizes lots of event spells, prescient is a reliable way to pull cards from the bin. Again, just look at every card that relies (or avoids) tokens in some manner and consider that Jacqueline now has an addition way to interact with these archetypes.
PS. Take 2 copies of Arcane Research if you feel like being a mad lad. Watch your friends get jealous as you get 2 additional xp each scenario and ascend to maximum wizard.
In the end her ability doesn't need to support a janky archtype (although it can, and does), instead your deck can also be "Mystic Goodstuff" with a powerful supporting ability for the team, and thats what I really like about her. Her ability will always be useful, it will have you discussing with your team each round where you should use it and where you need to be to help out. Its fun, cooperative, and powerful. If you made it to this part, I really suggest you try out Jacqueline Fine and see what its like to see the future and have (a bit more) control over chaos.