Do professors in your uni teach awfully if the class is directly related to their research interests? I noticed that both in my PDE class and in my difgeo one. Both professors had done their thesis and published many papers in their areas respectively and made hard concepts on other classes easy to grasp. But when it came to the area they're experts in, they both overestimated immensely how competent we were in the subjects, the PDE one went off topic 70% of the lectures on "interesting sidenotes" and the difgeo one assumed we could understand the Schoen-Yau conjecture in an introductory class. Aren't experts supposed to be able to convey ideas in simple terms? As mentioned earlier, they both are really good teachers on other classes.