"because Android 6 allows it to be formatted as part of the system storage."
Nope, marshmallow allows it to be used *instead* of system storage. You loose direct access to the relatively very (and in Samsung's case extremely) fast and expensive internal FLASH. You can choose to store some apps on the internal FLASH and that's it, data goes to the SD Card. The slow SD card will hurt performance of the whole phone. SD cards have no trim support either.
Adopted SD card storage in marshmallow is good for very low end phones which were slow anyway and have little internal FLASH to be wasted.
Biting the hand that feeds IT
