A great BW clerics deck is good at all stages of the game and on every axis. Your creatures are cheap and can grow into big threats, so you can race aggro in the early game and outsize Green midrange decks in the midgame. You can quickly stabilise by gaining life and creating a board stall and then pull ahead in the late game with card advantage and the ability to get in for the last few points of damage with life drain. You also have flyers, great removal, and both your colours have main deck able answers to the 2 almost unbeatable enchantments in the format - Roost of Drakes and Felidar Retreat. You also have some Party benefits. Black White can essentially be an aggro, midrange, and control deck in one deck, which isn't really fair.

This is a good format but imperfect - they gave Black White too many goodies and versatile cards, and made Green's payoffs too specialised, conditional and weak.