I would never say chemistry and biology are difficult to grasp. There's just a lot of content. A bit like Medicine. The content itself is fine. But there's so much.
Maths on the other hand... relatively small amount of content but the workload needed to grasp everything is a lot compared to other subjects. Especially M2, M3, S2 and FP3. Those exams are just disgusting. With maths there's no 'knowing the book inside out'. You just have to be talented and remain calm when you see weird looking questions.
On a semi related note, at Uni level, Physics is probably one of the hardest courses in the world.