The content isn't particularly difficult, it's just there's a lot to know - or should I say, memorise. Other subjects eg chemistry need a bit of memorisation, but often one concept can apply to many others. In biology there's a lot of specifics, and the harsh mark schemes reflect this.
In class, Biology is the subject I understand the most.. but in exams I do better in Chemistry, even if it's only by a few UMS. This amuses me, because I learn the textbook to the point of being able to recite information off any page for bio, but in chemistry I was still flapping about not knowing some key stuff half an hour before the exam.