Basically, you're going to lose assist and BMW bluetooth if you relocate the head unit. Relocating the head unit is a way to keep the HU on the data bus, so that if the car goes in for service, the dealer computer doesn't "see" a missing head unit, or error codes for assist and bluetooth not working - and then pukes all over itself when it tries to do a software update. Retaining aa hidden head unit keeps all that working.
musicar northwest portland or musicarnw.com
BMW systems and subwoofers