I studied this extensively during my phd. There is real evidence that the ratio of omega-3 to omega-6 fats in the diet has measurable impacts to health outcome

I studied this extensively during my phd. There is real evidence that the ratio of omega-3 to omega-6 fats in the diet has measurable impacts to health outcomes. Unsaturated fats (those which contain carbon <-> carbon double bonds, rather than each carbon being "saturated" with 3 hydrogen atoms and a single carbon <-> carbon bond) are classified by where that double bond takes place. The alpha carbon atom is at the carboxylic acid end and the omega carbon is at the end of the "tail". Omega-9, Omega-6, Omega-3 are telling you where the double-bond exists, relative to the end of the tail. These molecules can have slightly different behaviors inside the body, despite being highly similar. The reason that fish oil supplements sometimes show positive health outcomes is because they are high in omega-3s and help bring balance to the overall diet of people who eat an omega-6 heavy diet. People whose diets aren't overbalanced towards omega-6 don't see any positive impact from fish oil supplements. All that said, the impact is quite small. Avoiding seed oils often has the practical impact of avoiding ultraprocessed snack foods or shifting people's eating away from packaged foods towards fresh, whole foods, which have numerous positive impacts (less sugar, less microbiome-disrupting preservatives, less artificial colors, more fiber, more vitamins, etc) and so at population-level it can work as a reasonable proxy for "healthy" versus "unhealthy" food choices. TL;DR - seed oils have a sub-optimal ratio of omega-3 to omega-6 fatty acids. People should try to eat an overall diet which gives them a higher omega-3 to omega-6 ratio than seed oils will provide. Seed oils can be an acceptable part of a diet as long as this ratio is balanced by other meals. Overall, the omega-3 to omega-6 ratio is a relatively minor part of health, and a focus on sugar, total calories, sugar, fiber, sugar, protein, sugar, vitamins, sugar and minerals is the more important thing.