
Useful, but hard to deckbuild. slots, while not as contested as, say, deckslots, dont have too much space for tertiary strategies. If you do afford the deckslot for "Fool me once..." its a useful but not terrific card. Diana Stanley loves her some cancels though, so keep this in mind for her.
The mechanics speak for themselves, (although keep in mind that the card is played on treacheries -while they are being discarded-, keep that in mind if you want to hit something like Frozen in Fear), but the inability to surgically hit whichever treachery you like is a drawback. Think of it this way: Sometimes you play Ward of Protection against Rotting Remains to cover someone for whom a failed check equals defeat, but usually you dont block a minor treachery like Rotting Remains at all. "Fool me once..." doesnt have this precision. The most bang for your buck is hitting treacheries that are universally bad, Ancient Evils for example.
Obviously a greater playercount increases mythos drawspeed, so the usefulness goes up a bit, but in this case the chance of you personally encountering the desired target (a requirement for playing "Fool me once...") goes down dramatically, so that in of itself is a problem too.
"Fool me once..." is'nt bad, but it's largely inferior to similar mechanics and it's costing space and resources in a faction that often struggles to generate resources and cards. You will be forgiven for not ever trying this thing out.