i have already prepared (and currently am preparing) students for this type of exam. naturally, the difficulty of a task mainly depends on the student. what one finds difficult the other might find easy.
however, i can say that the difficulty in the written part is that there are various types of compositions and you need to master all of them. writing reports and essays can be very tricky because they are not so common in our everyday life.
i would say that the reading part is the least problematic.
speaking can be tricky as well if you are not used to being asked about various topics in a short time. but you can certainly prepare for it which makes it easier. some topics require mental maturity as well as language knowledge so you'd better research some questions that might arise.
the listening part can cause problems if you don't practise enough. though i must say that i really like the variety of tasks - note taking, multiple choice, short everyday conversations. you don't generally find these in other exams.