As I asked before, is there comparative data for pass rates and higher grades at these schools
It's interesting how the goalposts are moved.
iGCSEs are adopted by private schools. The heads assure the parents that this is because the iGCSE is more rigorous and demanding and offers a better preparation for A Level, a glossier coat for your horse and a wetter nose for your dog. Of course, say the parents, this is self-evidently true, and no-one demands to see GCSE results (shouldn't they drop, if the exams are tougher?) or the A Level results (shouldn't they rise, if the preparation is better?) The horses' coats and dogs' noses equally remain unaudited.
iGCSEs are adopted by state schools that are flirting with the floor standards, and a switch to iGCSE appears to often precede or follow being placed into a category. Observers suggest that it's a bit of a coincidence that many hundreds of schools, few of them with stellar results, a lot of them without sixth forms, should be switching, and suggest that the reason might be that the exams are easier. Suddenly, evidence that isn't asked for to support the proposition that they're more demanding is required for the proposition that they aren't.
Where's the evidence that the iGCSE is "more rigorous?" There isn't any, apart from some private schools who may have all sorts of agendas moving to them, and providing no evidence other than assertion.
Where's the evidence that the iGCSE is easier? A rather larger number of schools, most of them with challenges, moving to them.
Which you choose to believe is up to you. Neither argument has significant evidence beyond the behaviour of schools.