I've used the MSFs for a few years now, but not everyday. and so far I haven't had any problem with them.
I don't know exactly what the ingredient(s) is(are), but it could be some kind of 'mineral'...or maybe the MSFs themselves are fine with your skin, but when they are used/layered with the other face products on your skin, it might cause a chemical reaction or something... I'm only guessing...so am unsure.