momentum + Immaterial is a a bit OP for just being able to turn into that after one turn. You need to either kill it, or poison it on the turn it is played to be able to counter it. Sure, you can put Immaterial and Momentum on any creature, even something like lava golem, which is pretty powerful, but requires a bit of setting up to do. this card, just play it, next turn, quint, boom, you've got a powerful, unstoppable creature. A much easier, much quicker setup since you are only relying on two cards instead of three. Just too powerful with that momentum being added on.