Each academic writer has his own line of specialization that makes him an expert on the topic.
I wouldn't ever presume to consider myself an "expert" in any field, because even professional degrees, such as law and medical degrees, only really confer competence and a general understanding, albiet much higher
competence and understanding than someone without those degrees. Real "expertise" is something only acquired by professional specialization in some much narrower specific aspect within one's degree area, such as lawyers who specialize in corporate restructuring or 1st Amendment issues and doctors who specialize in pediatric oncology or knee and hip replacements. None of them would consider himself an "expert" in other areas of his respective profession just by virtue of his degree. Moreover, some of the project areas in which I'm most capable include Nursing and Political Science, despite the fact that I never took a single Nursing course or pursued a degree in Political Science, just for two examples.
However, even expert writers in the STEM field tend to think twice about writing a paper for a student when it involves heavy reading and creating an acceptable conclusion.
When it comes to high-level projects completely outside of my own degree areas, I'm still able to do them well enough that even my PhD clients are thrilled with the results; however, I may have to depend on them to provide the necessary sourcing for those projects. Typically, the combination of my writing skills and the client's much better familiarity with available sources results in much better results than the client would ever have been able to produce on his own using the same source material.