You can't just pigeonhole fats into three categories and be done.

Butyric acid and stearic acid are both saturated fatty acids, but they serve very different roles in the diet. Similarly, ALA and LA are both essential unsaturated fatty acids, but they compete for the same desaturating and lengthening enzymes in the body. So if you eat sufficient quantities of those fatty acids in the wrong ratio, you can still see symptoms of dietary deficiency. And different people produce different quantities of those enzymes or different variations with greater or lesser effectiveness.

The AHA made insufficiently informed recommendations, and the nonscientific population followed them, often by replacing animal-based fats with vegetable-based fats with vastly different fatty acid ratios.

These were sometimes chemically treated to turn them into trans-fats, which in a key-keyhole model of body chemistry is like bending a kink into the key to your front door, then jamming it into the lock with a hammer and forcing it with vise-grips every time you wanted into your house. The trans fats resembled saturated fats enough to be used in the same way, but that kink in the key would cause persistent damage.

And foods manufacturers also replaced fats with sugars and sodium salts, which caused different problems.

Different foods have different fat profiles, just as they have different protein profiles. Eggs and milk have amino acid ratios that very closely match what humans need, whereas beans and rice are insufficient in isolation, but complete in combination. Similarly, beef suet, pork lard, olive oil, coconut oil, and soybean oil have different fat profiles.

We still don't know what the "best fat" food is, like we know that poultry eggs are very nearly the "best protein" food. And "fat quality" might not even be as homogenous between individuals as it is for the amino acids.

Any recommendations at this time are almost certainly unfounded or unsupported by rigorous and repeatable research. You really have to do your own homework on this one, and avoid making any conclusions based on insufficient evidence.

> There is probably no harm replacing saturated fat with polyunsaturated fat, but that's not what happened based on decades of AHA and govt. recommendations.

One of the leading hypotheses is that omega-6 fat - which is a subcategory of polyunsaturated - is very very bad for you. So there is absolutely potential for harm in replacing saturated fat with polyunsaturated fat.