I think Jacqueline is an enjoyably straightforward character. Her limited card selection isn't that bad since most of the spellcasting decks I make are very heavy on Mystic cards anyways, and having the 3 for her intellect is the best secondary statistic for a Mystic.

I ran a mathematical evaluation of her special ability (arbitrarily assuming the value of tokens in the bag was +1, +1, 0, 0, -1, -1, -2, -2, -2, -3, -3, -4, -4, -5, auto-fail), and how it compares to just having a flat skill bonus:

1. If the check is beyond her skill level completely, obviously her power is useless and you would be better off with a real skill bonus.
2. If the test is doable, but challenging, that is where her power really shines and becomes very useful. Her power never becomes as useful for succeeding as a flat +2 skill bonus, but it gets close (and it could be better if your goal is to avoid drawing a specific chaos token at all costs).
3. When the test difficulty drops to the point where it would be quite easy without her power, with only a few tokens in the bag that can fail, her power starts to become less useful. If the only way you could fail normally is by drawing the tentacles token, her power is barely useful at all. There are likely to be token distributions which cause her power to actually increase the chance of failure, but that did not happen with my sample token bag (her power was still a slight net positive, the triple chance of drawing the auto-fail was fully compensated by the excellent probability of succeeding even if she does).
4. When the test becomes even easier, her power then starts to become useful again. It is possible for Jacqueline to reach a point where she is so skilled that there is zero chance of failure, something which no fixed skill bonus can ever achieve.

There are two main benefits of note with the Backpack upgrade.

The first and 'lesser' benefit is the cost reduction, and even that isn't insubstantial. Going from 2 cost to 1 cost is only 1 resource saved, but it makes a huge deal for characters who want to set up on turn 1. Having 4 Resources to work with instead of 3 means you have that little extra wiggle room to do something like play both Lockpicks and Switchblade, or set up two Hawk-Eye Folding Camera in one turn. It also makes the Backpack a much more attractive option for those who plan on cycling through their deck by finding and using another Backpack - 2 resources for a deep search is a much lighter cost than 4 resources, and those 3 resources remaining can help you get out one more play. It even serves as a reasonable card to draw when you're running low on resources - since you'll usually have your upkeep resource to use, and if nothing else, this will help thin your deck down afterwards.

The second and more notable benefit is that you're now able to search through 12 cards of your deck. At the start of the game, that means you're dipping through about half your deck, which is a very effective way of finding something useful. Even Prepared for the Worst only digs through 9 cards, and search through what's often a smaller subset of Items. This massively improves the reliability of the action. And, while it won't quite be as available as using Stick to the Plan for PftW, this has the potential of landing a fairly reliable Ever Vigilant. This becomes a tempting option rather than trying to buy a second copy of a high-XP card. Even before that point, this can boost reliability when only using 5 or 6 other item cards total, allowing for use at much lighter item counts than the base version.

Digging deep also means that you end up with addition synergy on Research cards. Effects like Astounding Revelation and Surprising Find suddenly become much reliable. With the recent Taboo of Mr. "Rook", the cost of repeatedly searching cards becomes higher, and this can be a reasonable substitute or convenient earlier step. It also has the advantage of using a common tag, meaning you aren't forced into inconvenient build plans in order to trigger them. It's not enough to trigger every Research card on its own, but it's something that can easily be slotted in when you're already using them.

And since I'm talking about Research cards, I'd be remiss to not mention the other obvious Seeker synergy, Segment of Onyx. If you're playing with someone who digs for those segments and manages to use up Pendant of the Queen, the segments all go back to your deck. And guess what can now dig through a 12-card deck and potentially throw out three of them for a single action, as long as you can afford the cost.

To summarize, in common circumstances, the low XP cost and low barrier to use makes this an incredible piece of added reliability. Many non-Caster characters will tend to have at least a handful of items that they want, and many of them also lack anything important occupying their body slot. If you value reliability over power, this is worth a look - and as campaigns go on, that can definitely be the case.

At this moment i think the safest way to use these -asset spells (alongside Eye of Chaos and Shroud of Shadows) is with Luke Robinson. The most effective usage of these spells comes with drawing one or even more -tokens. This comes with the advantage of getting bonuses even when missing the skill check, but also with the disadvantage to have to deal with an additional modifier of -2 (-4, -6, and so on). Therefore you want to draw -tokens but also need a way to compensate their drawback. So why Luke Robinson?

First of all every mystic can make use of Favor of the Moon to guarantee the draw of at least one -token. To compensate the penalty you can name this token with Recall the Future. Or as Luke Robinson you can pray to your Blasphemous Convenant to turn this one in an actual bonus to your willpower (combined with Recall the Future to +3 bonus). As an additional bonus the token goes back in the bag.

But how to generate these -tokens? Sadly mystics do not have a lot of ways to generate them. Promise of Power is a great skill card and highly recommended with this cursed style of play, but fortunately Luke can use Deep Knowledge to draw the needed combo pieces and fill up the bag. Furthermore he can use Stirring Up Trouble in high shroud places to flood the bag with these nasty tokens and get some easy clues. Fey is another skill that functions quite well with these kind of spells.

In the end it can only be said if you draw Dread Blessing as your weakness, you could actually call yourself lucky!

Incredible in Carcosa.

Picked up an annoying weakness that's blocking your infinite deck shenanigans, but it's labeled as a 'treachery'?

Buh-bye.

Present & future list of cards this skill may apply to (Spoiler Warning):

arkhamdb.com search

Pointing out potentially obscure interaction (if it's already been discussed, my b):

The ability on "Fool me once..." is entirely optional. The means you can lock away nasty treacheries like Ancient Evils forever to decrease their appearance from the encounter deck at all. Then in the final stages of the scenario you can always choose to cancel it anyway if you'll finish before reshuffling the encounter deck.

The reason I'm bringing this up at all is because my group has found that Syzygy is just an absolute nightmare to handle. Since it is a Peril, your Ward of Protection or A Test of Will packing friends won't be allowed to cancel it. If you have a way to ensure that you're the one to draw Syzygy (say... First Watch or Scrying) you can ensure one or both of them are completely removed from the scenario for its duration.

Notably the reverse also applies. "Fool me once..." cannot cancel Perils drawn by your allies despite its card text.

"Peril is a keyword ability.

While resolving the drawing of an encounter card with the peril keyword, an investigator cannot confer with the other players. Those players cannot play cards, trigger abilities, or commit cards to that investigator's skill test(s) while the peril encounter is resolving."

Anyway, there's plenty of encounter sets that include only one or two copies of completely disgusting treacheries that can be completely negated for the remainder with some luck and/or some encounter deck manipulation. Give it a shot!