Isn’t keeping 7 with this better than mulling to 2 with this in pretty much all cases? I don’t think mulling, with the intent of casting this spell, gets you any closer to finding something with it. The top cards of the deck are unknown in all cases.
First time this effect has been printed at 1 mana cost, and we all know how good 1 mana cards are. You can see at its ceiling it has greater impact than Bolt/Path to Exile and sometimes it has double or triple the impact when you're talking about interaction in a Planeswalker based format. If your opponent has 2 walkers and both players have a creature, this for 1 mana destroys two walkers. If you use it conjunction with sacrifice effects such as Cabal Therapy it just gets so much better. It could be up to a 1 for 5. (their creature, a card from their hand, two planeswalkers, and a Young Pyro elemental token)
There aren't too many 1 mana cards even capable of producing the same output. It is "Change of Heart". It is a ban worthy effect in most TCGs.
The fact it doesnt steal creatures greater than three is a limitation and that probably makes it more limited unplayable than the hyper efficient constructed formats that are choke full of amazing 2 and 3 cmc creatures.