Cancelling nasty treacheries is extremely powerful, especially for free, and especially especially when the only consequence is you having to do a bit more of something you're probably specifically built around doing. Hell, if you're running Milan Money or similar incentives to investigate, you might even "break even" in terms of the action cost of putting down the clue.

It strikes me as particularly good for Joe Diamond in the main deck, as his big weakness is treacheries.

Fast, 0 resource, Cancel a treachery card, costs only 1 xp.

Wow what a great card for seekers. The drawback is small(let's say it cost you 1 action to investigate).Even Roland can make a great use of it since he can add clues while slaying some monsters.

Like ward of protection and test of Will this card is an auto-include to avoid the terrible things that Mythos offers you and most of times punishes you.

There will be scenarios where its downside won't even be a true cost. I've been playing dunwich right now and in most of scenarios you don't advance the act by collecting x clues, but instead use clues for other things or just have to clear some specific place to get to the next act, so you won't even lose one action by dropping clues. Fantastic card!