Personally, I think griffins are quite well-balanced. Moreover, their roles are very diverse. IMHO, one of the most interesting upgrades in the game, with stats that are completely different; essentially, they are different creatures altogether. Some are simply avid attackers with low HP, while others are ideal for defense; attacking them more than once is scary. By the way, Battle Griffins are the only upgrade in the human city that is not focused on attack but on defense.