It's mostly bullshit.

Eat a balanced diet and stop stressing about specifics. Eat meat sparingly. Don't drink at all and it'll make a bigger difference in your life than seed oils. Drink in moderation if you can't handle that. Have fruits and veggies daily, as fresh as you can get them. Walk a few miles a day.

All the basic easy stuff is the most effective. There are no tricks, shortcuts or gotchas.

They're higher in inflammatory fats (most seed oils are predominantly composed of omega-6 polyunsaturated fats/inflammatory fats). While our bodies do need some inflammatory and anti-inflammatory fats (sort of like we need Cholesterol) we do not need too much. Calling them toxic is a step too far, but all foods should be consumed in moderation.

https://www.matherhospital.org/wellness-at-mather/diet-nutrition/the-connection-between-diet-and-inflammation/

Nothing wrong with them, but a lot of "alternative medicine" special interest groups wants you to believe there is, as evidenced by a lot of the links uncritically posted in this very thread alone.

A EXTREMELY helpful article, with scientific references covering the entire issue much better then I did below: https://www.dietdoctor.com/low-carb/vegetable-oils

Original reply follows - - -

The main issue is oxidative stress caused by Omega-6 fatty acids. Omega-3 are anti-oxidants and reduce stress. The ratio of o3/o6 is helpful in seeing how it can impact your body.

There is a small, but growing, body of research linking consumption of seed oils with metabolic problems (diabetes)

https://www.youtube.com/watch?v=7kGnfXXIKZM

I recommend this whole playlist

For people who are concerned about oxidative stress, they tend to cook food in fats, and avoid seed oil issues all together. Olive oil is a good seed oil, but getting a unadulterated pure source is problematic.

It depends on the seed - canola oil is known as rapeseed oil here in the UK and is good for you.

What you want to avoid are ultra-processed foods that tend to be high in sugar, salt and bad fats.

https://www.jeffnobbs.com/posts/death-by-vegetable-oil-what-the-studies-say

I found this post immensely helpful in describing the science behind the health effects of seed oils. But it remains to be said that saturated fat doesn't seem like the best thing for you either. I use the mostly unsaturated oil olive oil nowadays.