If I recall correctly for Onua vs Makuta, Onua got hit because Makuta striked him from behind, and this was a surprise attack. Since the Hau does not protect against ambush, Onua was sent flying. Based on what I have read in the Bionicle story, I agree that the Hau is simply a very powerful force field, which can nevertheless still be shattered against a very powerful blow, for example against the Bohrok-Kal. Tahu's Hau Nuva was unable to block a particularly devastating Kal attack. It can also be disrupted, for example Tahu activated his Hau Nuva in Karda Nui but one of the Makuta shattered the shield by using one of its powers (I forgot which Makuta and which power). Tahu also encountered leeches in the Karda Nui swamp which actually fed on the shield.