I'd say it's a matter of what are the chances of bad advice. If something people on the internet are talking about is widely known, there would also be papers with research, and I'm better off reading that. If it's a niche stuff, their talk on the internet doesn't bear statistical significance anyway.
Sure, there are scammers and incompetent doctors, but I would rather ask several people with a medical license, than several thousand laymen that think they are competent enough to give advices