** It is possible for the RAID header to cause misalignment of sector writes on RAID 1 by starting the array within a sector on an actual drive, such that manual correction of sector alignment at vdev creation does not solve the problem.
* 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. e.g. If you have arrays A, B, C and D; array B dies, arrays C and D
might be renamed to look like arrays B and C respectively. Giving the drives directly to the OS allows this to be avoided via naming that maps to a unique port or unique drive identifier.
* Controller failures can require that the controller be replaced with the same model, or in less extreme cases, a model from the same manufacturer. Using ZFS by itself allows any controller to be used.