You draw an epic amount of cards and do an epic amount of damage as more players are added to the game
Sure, you can combo it with cards that put extra counters, but if you can dedicate that much resources to one card, you can dedicate those resources to other ones that win you the game on their own, rather than relying on a combination of cards
All you have to do is cast spells on your opponents turn, which is quite easy for a deck with lots of Instants and Creatures with Flash
Damage wears off, and "this turn" and "until end of turn" effects end.) At the beginning of your next end step, you lose the game
When it's restricted, do you really plan for it
Harmonic Prodigy is high on our list for one big reason: it works incredibly well with Burning-Tree Emissary, and Burning-Tree Emissary is already a staple of the Historic format