I think the reason writers are often given conflicting advice is that writing is not an exact science. In math, 2 + 2 always equals 4 (how boring). I guess that’s one reason I love the Christian writing profession. There is so much room for creativity and literary license. Thus, there may be several ways of doing things, and none are necessarily incorrect.
I believe one reason those trying to teach writing go wrong is they think it’s” their way or the highway,” and they often stifle budding writers’ creativity. What has worked for them may not work for their students. As has been mentioned, they need to differentiate between what is their opinion and what is a hard and fast rule of writing. I know I’ve worked with students who have been given information that just didn’t work for them.
However, I’ve also seen students given blatantly incorrect information. Since I tend to be brutally honest, I struggle to correct the mistake without calling down the “professional” responsible for the misinformation. Yet, at the time, I feel like screaming and tearing out my hair.
Bottom-line: We, as professional writers and editors, need to keep abreast in our ever-changing craft and carefully consider the information we pass onto our students, conferees, or friends.