"Fool me once..." offers a similar promise to Mystic staple Ward of Protection of protecting you and your party from dangerous treachery effects. I say "staple", but actually I've recently gone a little cold for Ward because it turns out taking a horror along with spending a card and a resource is a fairly steep price against most treacheries, although it sometimes is worth it to protect vulnerable teammates or to just cancel the harsher treacheries, such as Ancient Evils. How does "Fool me once..." compare?
"Fool me once..." still costs a card and a resource, and while it doesn't deal you horror out of hand like Ward does, it does demand you actually resolve and discard a treachery in order to prevent a future copy of it. Now, you'll still count as having resolved the Revelation ability on a treachery that you passed a skill test for; the good news is in theory "Fool me once..." sometimes costs you merely a card and a resource, but this means you'll have to suffer at least one Ancient Evils in order to cancel the next one. Additionally, this card demands that you resolve the treachery to be cancelled, and not a teammate - so you have to draw it before they do if your intentions are selfless.
On top of that, not all treacheries with dangerous Revelation effects actually discard themselves until they're already done hurting you pretty bad, such as Dunwich encounter spoiler and Circle Undone encounter spoiler. Ward of Protection does better against those, and is generally much more flexible in what it can cancel. It has been pointed out that you can "trap" a treachery attached to "Fool me once..." by keeping it in play so that it doesn't shuffle back into the encounter deck, but unless you were highly likely to draw that treachery twice again during the scenario, you were just as good if not better cancelling a second copy with "Fool me once..."'s reaction.
So, when is "Fool me once..." good? I think much like Ward of Protection, this card isn't going to be something you spend experience on for all investigators for whom it is available. You ought to have a good idea of why you're buying "Fool me once..." and the kinds of treacheries you intend to use it to fight. You might for instance take this if you or a teammate is particularly weak to willpower treacheries. Notably, "Fool me once..." is likely to overperform in The Forgotten Age (perhaps this is being alluded to in the art?) due to the Explore mechanic allowing you to dig for treacheries early in the scenario.
Additionally, the more teammates you have, the greater the likelihood that the same treachery is drawn repeatedly before the scenario ends. In a 3 or 4 player group, "Fool me once..." goes especially well with First Watch and "Let me handle this!" to really control the Mythos phase and improve the odds of you drawing the treachery you want to cancel before someone else does. Obviously this synergy is going to be most effective in investigators equipped to take on multiple encounter cards, which typically means those with high willpower - so Zoey Samaras, Leo Anderson, and particularly Diana Stanley who loves that magic word "cancel".
Even if this card is as situational as I believe it is, it's still nice to see the Guardian archetype of protectively tackling encounter cards continue to be fleshed out, and I'm definitely a fan of the concept and theming.