We had the same issue with Agent Orange in VietNam. Eventually it became so political that the government yielded and recognized a set of conditions that would become the basis for a service-connected disability. There was some basis in science but not much.
I think that agent orange was a legit issue. I’m not sure if there is any good way to measure the amount of exposure though. As far as the burn pits go, same thing. No way to know what was being burned on any particular day or which way the wind was blowing.