RAID requires a set of identical disk drivers in a single RAID group; you would face penalties if you change the disk size, rpm, or disk type. Doing so would adversely affect the capacity and performance of your storage system. This makes RAID highly choosy about the hardware.
Also, enterprise RAID-based systems often require expensive hardware components, such as RAID controllers, which significantly increases the system cost. These RAID controllers will become single points of failure if you do not have many of them.