Yeah, most modern diet stuff is crap. As a general rule, anything which would have been available to us naturally before everything became processed is probably not bad for you (in moderation), because it's what our bodies were designed to live on. Of course, you can't make money telling people that so they come up with all sorts of crazy new fads to sell to people who are unable to see through it. I used to read a lot of nutritional advice but stopped because at some point you realise that they're always just trying to sell you something new, and after a while everything you read contradicts something else you read.
Regarding animal vs veg fat, I watched a YT vid a while ago which said that the ones which are bad for you are the ones which oxidise easily. IIRC certain seed oils are the main offenders. They're also the ones which involve the most intensive extraction processes, which ties in with what I said about stuff that occurs naturally.