You know...it really depends on the person.
I found chemistry severe...the organic chemistry is a kiler!
I've been finding biology ok, it don't love the subject but I don't really find it that hard, yet my friend who is hoping to get an A* in chemistry is really hoping she can get a B in biology.
I always think it's like this (I'm not sure about physics)
The content in chemistry is very quite difficult to grasp, learn and understand. However, if you know and understand the content, in the exam you will be fine.
However the biology content is easy, yes it's a fair bit to learn, but it doesn't take that much. However the tricky bit is the actually exam questions, you can completely understand the content but some of the questions are just like... what the ****...?