Most of engineering theory courses will be considered difficult because it requires rigorous thinking processes to solve problems. And their difficult level can be more depth than calculus which is general requirement of engineering students to take.
But beyond the engineering courses, I think the most difficult courses engineer students encounter but very often ignore is English composition classes.
Majority of the time, engineers spend 50% of time writing emails and reports from their daily tasks. If your English writing is lack, your career will not get further. And the most difficult of English composition course is that engineer students don’t contribute enough time to study and learn compares their engineering courses.