So I think there's 3 important things to note about this card that you might have skipped over if you only gave it a cursory glance.
First - the will-check replacement is a reaction triggered ability, meaning you can use your will for ANY check. This makes it markedly different from your normal fist check Shrivelling or Wither which can only be used on a fight action but cannot be used to open a locked door, your normal book check Rite of Seeking or Sixth Sense which can only be used on an investigate action but cannot be used on book check parlays or when you are using an investigate action that is printed on a location or the act/agenda, and your Mists of R'lyeh which can be used to evade but not to beat the evasion check on encounter cards.
Second - this asset takes up BOTH arcane slots, so unless you're using Sign Magick, you won't be able to have this card and one of the more specialized cards mentioned above out at the same time.
Third - the card is Myriad, meaning when you get it, you're allowed to put 2 more copies of it in your deck for free (so potentially 2xp total for 3 of these guys).
So is this card good? Eh... probably not. The 2 arcane slot usage makes it very difficult to put in a deck. Ultimately once you're in the lategame, Mystics are going to be relying on the powerful effects that their upgraded arcane slots bring. From Shrivelling you get 3 damage attacks with a +3 will on your roll, and with Sixth Sense and Rite of Seeking you're looking to get multiple clues with one action with a +2 will on the roll. Simply swapping out your fight or investigate action with will without any bonus to the roll really hurts (though granted you'll probably have a Holy Rosary and possibly Four of Cups to mitigate against the need for the bonus) - and not having any bonus damage or clue pickup per search makes the card lackluster.
Now of course, the benefit I discussed first, that it can be used in any type of check is immensely powerful. But Mystics already have good tools for dealing with these sorts of checks. If the check is coming from the encounter deck, you can just cancel it with a Ward of Protection (or the level 5 version Ward of Protection if you're playing solo). Otherwise you can fix the token bag for non-investigate or fight action skill checks with cards like Dark Prophecy or Defiance or Counterspell or even Seal of the Elder Sign if you're feeling spicy - and this is not to mention that often a bad roll can be undone with Time Warp or the effect ignored with Deny Existence. So you're paying a very hefty price (the loss of the bonus will to the roll, as well as the extra damage or clue benefit discussed above) to cover a weakness that Mystics already cover very well anyway.
The other issue with it is its limited usage. It's got 3 uses. It tries to make up for this by being a myriad (meaning once you've got 1 on field, you can still have 2 in your deck or hand), and allows you to discard them from your hand as a free action to put another 2 tokens on. If you go with that option, that's 7 uses. That's it. 7 uses total, and it cost you 3 cards! And the entire time it is up, it is hogging both your arcane slots, so you're using it for every check - you can't leave the fight and investigate checks to the specialized arcane spells because you can't have those out! So these burn off pretty quickly. Of course, you could replay them instead - but in that case, you're paying 6 resources and 2 actions for just 2 tokens - so that's almost never going to be worth it.
That said, I can see some use if you are planning on running Sign Magick x2. If you can keep this card in reserve so you only have to use it on the checks that your more specialized cards can't do, it can probably be used as an alternative to all the cancels I discussed above. But the setup is harsh - 9 resources to get both Sign Magick and this card out, and your hand slots are gone (so no The Chthonian Stone or Ritual Candles if you were planing those, and no Twilight Blade - not that Diana Stanley would ever want to get rid of the cancels anyway).
The other potential benefit of this card is that it can act as a buffer card if you're having trouble finding your spells. We've all had our Shrivelling and Sixth Sense at the bottom of the deck, and having a myriad card like this with 3 copies to give us the willcheck substitute we need in a pinch has undeniable value. The issue though again is that it hogs both arcane slots, so once you find one of your specialized cards, you're going to have to discard this card to make room for the specialized card if you want it out on the field. This a problem because that means it will fail to substitute for the second card (for example, if you find your Shrivelling first, then you'll have no way to cover investigate checks until you get your Rite of Seeking.) So it is really only an effective substitute so long as you have NEITHER of the specialized cards. So you're better off just using the tried and true Arcane Initiate initiate x2 to find your spells rather than rely on this as a stopgap. And to add insult to injury, this card only has a test icon of +1 will, so it's not even that good when committed to a test - so if you're using it as a substitute card and you draw it late, it's almost a dead draw.