Hi! I take iGCSE Edexcel Maths and Bio and CIE English Language.
Maths: I don't find the topics very challenging but there are the last 3 or 4 questions at the end of the paper which can be really tricky and need a strong conceptual understanding of topics and often an ability to accurately replace numbers with algebra in complex equations.
Biology: Edexcel igcse sciences are some of the highest level i/GCSEs you can take, and much of the bio course taps into A level content (especially protein synthesis). Overall I find the exams relatively straightforward and it's mostly about rote learning the key words, and the questions are fairly formulaic and repetitive.
CIE English Language: Oddly this is the most formulaic GCSE I take, and especially the entirety of paper 1 is just a set structure you need to learn to do well. I don't find it tricky, and my school had actually only started teaching us any English language only a few months before the actual GCSE. If you're good at English lit analysis and enjoy composition then you'll find it a breeze.