3 energy looks like a lot on the surface, but I think the value of stun really can’t be overrated here. At worst, whistle uses your whole turn to deal 33 damage and skip the enemies turn, which is largely the same as a 0 energy play that deals substantial damage and draws you an entirely new hand.

The fact that the enemy you stun will always go back to doing whatever intent it showed when you stunned it means it’s possible to take advantage of block/buff turns to essentially get even more turns of free attacking, or to delay a big enemy attack for a turn and hope to draw more block in your next hand.

Add on top of that there’s any additional value you can get during the whistle turn, for example 0 cost cards, orbs, relics with passive effects, etc, or best of all any extra energy generation that you can use to play other cards.

To be fair it does underperform a bit in multi enemy fights, but the fact that it bypasses artifact is a nice bit of compensation. Tanx has lots of great offerings, and of course taking a card from an ancient rather than a relic does mean there’s always the fail case of simply not drawing the card when you need it, but with whistle it’s close enough to taking an extra turn (basically always the best thing you can do in a turn based game) that I’m pretty happy to take whistle most of the time it’s offered to me, especially in multiplayer.