I can only speak for maths which is the only one they do at our school and the only one I've looked at.
The iGCSE covers harder stuff like calculus and the exam did look harder to me. But they don't have to do any coursework (which iGCSE proponents say is boring, time-consuming and easy to plagerise) so that frees up more teaching time to cover the extra stuff. So the exam is harder but counter-balanced by lack of coursework. It looks to me like iGCSE is a better preparation for A-level.
But if, as was alleged, the children don't actually get lower marks when they take iGCSE then I assume the thresholds are lower so that the exams have harder content but are not more difficult to pass. Does that make sense?