The Orcs bird cost more gold, lumber, and food(4)(the spear thrower if you know him), yet that Naga serpent fly can kill it on its own. a full army of night elf archer die against full army of this serpent fly. Try it on test map and see. Snap dragon also kills crypt fiend, and Reaver kill footman, and makes huntress hit points below 100 (around 92)
I made reaver weaker, snap cost more gold and lumber, and serpent fly cost more food, lumber, and gold.