If you are exporting from a site using AFC 3 or 4 and importing to a site using ACF5 you may have issues.
More importantly, image fields of any kind store the image ID values of the images. If the imported images don’t have the same post ID and they do on the original site, which is more than likely, then the images will be broken in the image fields. So basically the WP export import will never work with ACF image fields.
