I would say that the main factors in determining which subjects appear to be difficult, are the attitude and interests of the student.
Students who are good at memorizing facts will find Air law very easy. But those who feel the need to understand the why and wherefore of a subject usually find Air law very tedious, so they simply witch off.
Those who like mathematics and physics are likely to be very interested in subjects like POF, PERF, AGK and MET. But those who have a morbid fear of all things technical are likely to be overwhelmed by these subjects.