The existing reviews have already touched on the bonkers combo potential that this has with Double or Nothing. Indeed, it is the core (along with Quick Thinking) of the well known Rogue infinite actions combo. In spite of these cards being tabooed, it is still quite possible to go infinite, it's just easier for it to be disrupted by a . My tip, for those who want to engage in this degeneracy, is to take Swift Reflexes, as it gives you a bit of a buffer zone on this disruption, and Three Aces, which allows you to guarantee the full payout on (usually) every other test. Unfortunately, the resources and cards from Three Aces can't actually be doubled, but they're still good.
I wanted to add a few points on using All In that don't require DoN:
For one thing, All In allows you to basically draw a whole new hand if you want, while cashing in on various skills. This is actually pretty great even without DoN. You could quite easily get +6 or more to the test without actually depleting your hand, or with very minimal net loss of cards. That might even be enough for you to pass a test in some cases, while still profiting handsomely from succeed-by effects. This works best if you have a lot of s in your deck because you're basically replacing generically useful cards with other generically useful cards, while drawing through your deck. Did I mention that any weaknesses you draw get shuffled back into your deck? Yeah, it's really good.
Another point about All In is about its synergy with Ace in the Hole. Ace in the Hole is an extremely powerful card that gives you +3 actions! This can be clutch even if you only draw it once. Those 3 actions can be the difference between being defeated by some treachery and defeating a boss to advance the final act. If you have All In in your deck, along with a few other sources of card-draw, it is not unlikely that you can see Ace in the Hole several times in one scenario. I would even go as far as to say that Winifred Habbamock, with a high-xp deck, (going into Shattered Aeons, for example) that includes both Ace in the Hole and 2 copies of All In, feels completely busted. This is even true without DoN, but with, it's even more insane.
The other side of that synergy is that getting 3 extra actions can help you play or commit any excess cards you have in hand from All In, so that they don't just get discarded. That's generally the only drawback of using All In, IME.
The final point is that, because this card lets you shuffle back weaknesses, if you have a nasty weakness that shuffles itself back into your deck unless there are 5 or fewer cards left in it, All In can help you get through the part of your deck that is most vulnerable to repeated draws of that (perhaps between 5 and 10 cards). I'm not sure how likely you are to be able to actually wait to get to that point, but even without that benefit, this is still a good choice for Sefina, because she runs a lot of events, and so wants to keep her hand full, anyway.