Interesting to bring up morality with polytheism. Easy enough getting morality from one God.... How do you determine which is correct in a polytheistic system?
It's worth keeping in mind that the an argument concerning a single correct system of morality contains an assumption or preference. The lack of such a system (or the lack of ease in obtaining knowledge of it) is not necessarily something a polytheist would feel the need to respond to. Christians don't feel the need to respond to, for instance, the charge that Christianity doesn't tell them the right way to cook spaghetti, but they wouldn't say it's a fault of Christianity - they'd say it's irrelevant and wonder what you're on about.
A polytheist could along the same lines wonder why we're seeking a single correct system of morality. When, precisely, was this promised to us? Moreover, why should we expect it to be easily understood? Looking at the universe without the lens of a particular religion, it seems as though morality is meant to be hard, since there are so many ambiguous cases, even
with the lens of a particular religion. Not a few philosophers have suggested that the purpose of life is to find a
sufficient moral system.
This is especially true in historical polytheism, where the gods are not necessarily meant to personify the best possible. The early Greek atheists quite rightly pointed out that some of the gods were not worthy of worship, since some humans were obviously more virtuous (by any reasonable person's definition) than a philandering or murderous deity. In that case again, why are we looking at the gods for morality? When was it promised to us that a god will be the source? At best, polytheists said, a person should understand how to act so as not to anger particular gods, but this has nothing to do with some universal code.
(In fact, this is the way the large majority of Christians live in practice. I have noticed that the concern is not over the inherent correctness of an action, whatever this is supposed to mean, but the possibility that it will incur wrath if not simply hell.)
In the end, I would suggest that the idea of a revealed morality, both unambiguous and easily understood, is mostly a canard. Having observed conversations from all sorts of philosophies and faiths, the debates are always the same - there are a number of axioms and there is philosophical generalization. None of these axioms are self-evident, even if they happen to be contained in old books. The decision to take as axiomatic the Bible doesn't seem to be any more interesting than taking as axiomatic another text.