I think different people will find different subjects " harder". It depends on your strengths. My daughter did Physics, chemistry, maths and biology and found biology the hardest. Another daughter did maths, FM, chemistry and physics and found physics the hardest.
It seems that people automatically assume that STEM subjects are harder, yet it is far more difficult to get A* in history at A level than it is at maths. My youngest daughter is taking RS, German , history and maths. She finds maths the easiest and history the hardest, but her friend ( doing the exact same subjects) finds German the hardest and history the easiest.