Is the product, Navage, promoted for flushing out the nose with saline as a health maintenance strategy, most useful for treating nasal allergy symptoms, or is there a downside for this indication as well?

There indeed is a downside in the weakening of host defense mechanisms in the nasal passages through flushing away secretions designed for protection, and that includes the very mucus production providing a mechanical barrier to contact with the cells lining the airway for people exposed to inhalation of pollen they are highly allergic to. The trade-off might be desirable to flush such pollen accumulation away after a trip outdoors, for example, as a way to avoid symptoms. But that is a minority of individuals and it will be difficult to know when such an exposure has taken place. So there will tend to be an excessive reliance on nasal lavage for such sufferers. Some will end up with a net positive benefit, some not.