Polyunsaturated fatty acids Omega-6 and Omega-3 primary concerns are with hearth health. It's not that Omega-6 fats are bad, we need them and we need Omega-3's. But they the typical American diet provides an Omega-6/Omega-3 ratio of about 15:1, whereas our Paleolithic ancestors are thought have had ratios closer to 2:1 or even 1:1.
Since Omega-6's are mostly pro-inflammatory and Omega-3's are anti-inflammatory, a balance of the two acids is essential.
You can find more information in Loren Cordain's research at:
Oops. I can only post one link, but if you look for his paper 'Fatty acid analysis of wild ruminant tissues ...' and another: 'Realigning Our 21st Century Diet ...'