Hi,
What is difficult and easy differs from person to person.
Once you begin your preparation, you will get to know where you are facing the problem. For this question the solution lies with you only. Simply if I quote that I have faced difficulties with taxation subject, it is like giving you unnecessary block right from now itself that 'its difficult, its difficult" though you may or may not feel once you begin your preparation with respect to that subject.
But I can say that taxation and Law syllabus is huge comparatively. At the same time other subjects are no more inferior though!
Coming to your next question, probable reasons for students scoring lesser marks in Law could be of quoting wrong sections, filling answer sheet with irrelevant matter, missing important points that are for sure required for the answer, missing proper prinicple to quote when question is asked in indirect way. For all these, it is advisable to follow the practise manuals to know how the answer has been arrived in a step by step way with good presentation.