Ra should at least gain half of the total ATK of the monsters used for its Tribute Summon. Say, you tribute 3 copies of Blue-Eyes, it'd only have 4500 ATK while it's one of the BEST scenario. This means that 3 copies of Ra's disciple only give you 1650 ATK. Well, 0 ATK for the cost of 3 tributes really is a 🔥!