The difference is the connection for the SSD is Thunderbolt 3 and SD card readers are basically USB connections. Transfer rates are much much faster on an SSD. Also, you can obviously get much larger SSDs then SD cards. Why would you use limited in size ROMs? I can't think of a slower way to access data. Especially when you can get raid 5 or raid1 cabinets which provide a fair measure of redundancy and/or reliability.