* If multiple RAID arrays are formed by the same controller and one fails, the identifiers provided by the arrays exposed to the OS might become inconsistent. Giving the drives directly to the OS allows this to be avoided via naming that maps to a unique port or unique drive identifier.
** e.g. If you have arrays A, B, C and D; array B dies, the interaction between the hardware RAID controller and the OS might rename arrays C and D to look like arrays B and C respectively. This can fault pools verbatim imported from the cachefile.
** Not all RAID controllers behave this way. However, this issue has been observed on both Linux and FreeBSD when system administrators used single drive RAID 0 arrays.
One might be inclined to try using single-drive RAID 0 arrays to try to use a RAID controller like a HBA, but this is not recommended for many of the reasons listed for other hardware RAID types. It is best to use a HBA instead of a RAID controller, for both performance and reliability.