You generally wont have the mana available to proliferate until the mid or late game, but the baseline here is great The really cool thing he can do is allow you to play a creature with a tap activated ability and use that ability right away and then you can use the +1 to use the ability again
This is partly, because it allows exiling the specific card that you know you can play immediately, most often a land
(2022-04-29) If you pay the casualty cost of a spell, the copy will resolve before the original spell
It can't be saved by the +1/+1 counter that would have been put on it
As a product line, Modern Horizons has always shaken up established formats: from the originals Hogaak, Arisen Necropolis, which terrorized tournament play in 2019, to Modern Horizons 2s Ragavan, Nimble Pilferer, which still reigns today