Hi @wang,
for what I know it's a tecnical restriction: the chassis/controller of a FAS will contain different VPD (Vital Product Data) than the ones of a V-series machine, thus rendering the two incompatible.
Since these values are checked when the system boots up, there is a chance that mixing a FAS controller and a V-series chassis (or the other way around), will prevent the controller from booting.
Regards
Lorenzo