In a STEM class the goal is not necessarily “mastery” of the material, but a thorough understanding of at least some portion of the professors vast expertise. Of course this applies more to upper level classes. If your professor is up there doing a brain dump for 4 months and you grasp 60-80% of it well enough to answer test questions that is probably pretty good. I would rather be exposed to everything a top level research PhD knows than have him/her cap what he puts out there in an effort to make sure a high percentage of the class reaches “mastery” level.