The fully general argument against supernatural belief is physicalism/reductionism. The fully general argument against omnipotent/omnibenevolent beings in our local space is anti-panglossianism, ie fun theory. Pick one.

That said, when someone starts arguing that it's impossible to derive ought-from-is, and therefore we need God to ground morality, I always want to ask if it isn't a bit suspect that they just derived is-from-ought.