BRG relies on activated abilities from Gene Pollinator and Umbral Collar Zealot, plus the Lander, Clue, and Treasure tokens their creatures leave behind, and Clarion Conqueror turns all of those off while it's on the battlefield. The Sackville-Bagginses uses triggered abilities, so Conqueror doesn't stop it. Avatar's Wrath is a strong sweeper against their wide board of 1/1s and 1/2s like Greedy Freebooter and Callous Inspector. It exiles every creature except the one you choose (for example Clarion Conqueror), and until your next turn they can't cast spells from anywhere other than their hand, so they can't recast their exiled creatures for {2} right away. Your own exiled creatures can be recast for {2}. Springleaf Drum and Zack Fair are too passive when you need to stabilize against their aggressive creature curve, and both have activated abilities that Clarion Conqueror would turn off anyway.