It depends on the person studying it!
The topics I tended to find most difficult (as did a lot of my colleagues) were:
Information Engineering, which isn't as likely to be a course in itself but will be involved in an electronics or software course. Lots of maths in the frequency domain.
Thermodynamics, likely to crop up in Mechanical / Aeronautical - I found the concepts difficult to visualise as it's all heat flow.