When calling ionic resources on Windows from a terminal prompt all the images are correctly generated and config.xml is updated with the new image paths. But as soon as you want to build this on Mac, you get cp errors as Mac can't find these paths. Ultimately your app may be built with old images as splash and icon.
Reason for this is that when generating on Windows you get \ in the paths, on Mac you get /. As / seems to work on both platforms, ionic resources should probably replace \ by / on Windows.