If a writer has (a) qualification(s) in "A", it means that they know something about "A". It doesn't mean that they know anything about "B", "C"....."Z". However, humans being human, they have biases. Writers write in a biased way. Also, readers read in a biased way. Having (a) qualification(s) means diddly-squat. What does mean something, is backing-up what's written with quality evidence. As I have no formal qualifications in Diet & Nutrition, I try to do that as often as possible. When I don't, it's my opinion.
Many people are intolerant of other people's opinions. No wonder so many "fights" break out on forums, message-boards & blog comments. As a writer's qualifications mean diddly-squat, what's the point in arguing about a writer's qualifications? There isn't one! It's an ad hominem fallacy. Bloggers whose blog contents consist mainly of ad hominems & other fallacies are ass hats.