RAID 5 reduces or eliminates any performance gain due to striping if there is a substantial amount of disk writing. Multi-user disk striping on the other hand can be accomplished either in software or hardware. The hardware solution may be standard with some systems, though default settings may not be as desired. This becomes a liability later in the life of a system when new larger drives are available. More complex hardware means greater chance of board failure.
Software can use simpler standardized host adapters less likely to fail.
The presence of relatively large RAM buffers on the board jeopardizes data integrity in case of power failure, and is relatively expensive. It is harder to update board firmware when the manufacturer makes bug fixes. There is a presumption that a hardware solution must be faster than a software solution because fewer CPU cycles are used. The opposite in fact is often true. Dividing data between multiple drives takes very few extra cycles, so downloading does not matter.