Is his course difficult because of the subject matter or because he is a terrible teacher?
I remember several professors who spoke marginal English. I had others who made ideas impossible to understand.
Professor Jones was teaching Organic Chemistry. It is a premed wash out course for those lacking the skills and ability to succeed in medicine. It is a very difficult class for everyone. The “students” that rated him poorly that led to the firing were lazy whiners who felt he gave too much hard work and they were unwilling to do it. If they couldn’t do it, they have no business going in to medicine.