Many years ago there was some bad press here in the UK about many common cooking oils. The only one deemed safe at the time was pure Olive Oil. Another one added to the list was Avocado Oil. Butter and Ghee (clarified butter) are also regarded as good for cooking along with animal fats.
More recently all seed oils have been cited by some as being particularly unhealthy.
We’ve only been using Olive Oil or Butter for cooking for over 35 years now. Obviously when you eat out in restaurants you can’t really avoid seed oils.
When you get into the bioenergetics of exactly how oils are used by our cells, polyunsaturated fatty acids, PUFAs, or "seed oils", are pretty bad. These include the vaunted Omega-3, -6 and -9 (which really just refers to the bond from the methyl end of the lipid at which the first double bond occurs, there are actually many PUFAs that meet the criteria for each, and others.) They are very reactive with oxygen, and will often spontaneously oxidize even in your blood stream (thanks to all those double C-bonds!), releasing what are usually toxic metabolites. In cells, in the ER, their initial stages of oxidation can still sometimes produce toxic metabolites, and overall the metabolization of fatty acids produces more ROS (reactive oxygen species) as well. Saturated fats, like butter, don't oxidize arbitrarily, are far more stable, and generally require less complex metabolizing reactions requiring fewer enzymes than polyunsaturated (or even monounsaturated, which still has to go through a couple of extra enzymes and still runs the potential risk of arbitrary oxidization.)
Interestingly, the reason why polyunsaturated oils aren't really good for humans, is probably also why they are good for finishes. Linseed, walnut, tung, etc. oils are seed oils, thus polyunsaturated fatty acids. That means they have fairly long chains (carbon chains) with at least two or more double C-bonds, which is where the oxidation occurs, and which can serve as junction points for polymerization. Longer chain polyunsaturated fatty acids will usually have more of these double bonds, and should therefor polymerize more and with ultimately longer chains and more cross-linking.
I would still, however, want the oil to be refined and not contain any proteins. As they will break down, especially under UV light. I guess I can't say for sure how that might ultimately affect the finish, but I certainly wouldn't want to eat out of something that had a bunch of bound up fragmented proteins and broken down amino acids and such...
