Yeah, Karn is really good here. Got all my wins in no time with mono-Green Nissa-Karn-Ramp. Seems like a lot of people are just bringing in their Standard decks, which I don't think is that good of an idea.
Now to be fair, mono-Green Nissa-Karn-Ramp is also just a Standard constructed deck, but since ramp is already a central pillar of the deck, getting extra free ramp helps it a lot more than it does other decks. Aggro gets faster, but so do you, and they'll run out of gas much quicker than you. Karn also just shuts down your opponent's Treasures (and most format-specific gimmick decks along with them) and fetches you specific answers to their deck, setting them back massively if not outright winning you the game. Scapeshift barely benefits from the Treasures at all, as they'll still need to get seven Lands into play, and by that time I'd have two Amulet of Safekeeping on the field to shrink their Zombie Tokens to 0/2s.
Outside of ramp decks, I feel Feather is one of the few straight-from-Standard decks that really benefits from the Treasures, since they're less about early aggression and more about getting certain pieces on the board with protection up, which the Treasures allow them to do much faster.