The thing to remember here is, RAID is complicated enough that *nobody* really does it in hardware. It's all software (or flash-based software a.k.a. "firmware").

So.. would you rather trust your data and time to a single factory burn of the software, or rely on something that can-be / is bug-fixed as the need arises, a.k.a. "software RAID". And if the O/S does the (software) RAID, then it can also (in theory) do a superior job of scheduling the I/O to each individual drive (= faster I/O).

The answer may also depend upon one's choice of O/S.

Cheers