To be precise, I think that the negative impacts of the ILIS system are more than compensated by its positive impact when you actually need stabilization, i.e. when you are shutter speed-limited. Plus the stabilization of the viewfinder, especially in the case of long lenses.
But when shutter speed is not an issue (i.e. you have enough light that your shutter is fast enough for whatever focal length you are shooting at), I cannot believe that having an element moving inside the lens does not have a detrimental impact, albeit small.
I've read things about it but nothing overly scientific.
E.g. there's
this old thread on dpreview. Or I could find
this article.
I could not find anything more definitive than that