I think there are two questions here. The first (the OSC one) is about the helpfulness of a given writer's advice on writing fiction. The second (the Paolini one), if I understand it right, is about the "goodness" of a given writer's fiction, and (I'm guessing) about the usefulness of examining their fiction for the purpose of learning how to write fiction.
I'll tackle the second question first. If an author's work is popular, and you want to write popular fiction, then there is probably something you can learn from it. For me the example is Dan Brown. I find his writing awful to the point of annoying me. And yet whenever he releases a new book, I buy it immediately, and I can't put it down. So clearly he's doing something that works for me. Given that I would love to write books that people can't put down, I could probably learn something important from Dan Brown's books, regardless of what other people think of his writing. And regardless of my own annoyance at his authorial shenanigans. (If only I had the stamina to fight through my annoyance long enough to analyze what he's doing.)
As for the first question, about a given author's advice about writing, I think you can apply a couple of tests:
- How successful are the author's books?
- To what extent have other authors benefitted from this author's advice?
I would give more weight to advice from an author who has demonstrated success repeatedly over many years, and less weight from an author with one or two books, or with books with limited sales. Or no books at all. (Note that my interest is commercial fiction. Literary writers might weigh things differently.)
A caveat: People do not always know the causes of their own success. Their advice may (unbeknownst to them) rest on many assumptions and on conditions that are peculiar to them, assumptions and conditions that might apply to you, or might not.
Which leads me to my second test. Can you find other successful writers who attribute their success partly to advice from the author in question?
I know of numerous writers who attribute their success in part to workshops they took from Orson Scott Card. Brandon Sanderson, Dan Wells, Mary Robinette Kowal, David Farland. And several writers in turn attribute some of their success to things they learned from David Farland.
All of those people have achieved significant success. Of course, the caveat above applies here as well. Their success doesn't prove that OSC's advice is any good in general, or that it is right for you in particular. But it's gotta count for something.
A final note. A single Stack Exchange contributor's opinion of a the worth of a given writer's advice does not imply that the writer is generally not respected. That contributor (whose respect around here is enormously well deserved) was careful to phrase that opinion clearly in the form of a personal opinion. If you get a lot of opinions like that, that adds up to "generally."
Note that I am not commenting on the wisdom of that contributor's comment. I am cautioning only about treating any one contribution (no matter how wise) as "generally."
OSC has said things I abhor. Those things are outside the realm of advice to writers. So I weigh his advice very differently depending on the domain of the advice.