That is not correct.
On a hardware RAID card, the firmware on the card runs on a processor on the card, hopefully with a RAID-acceleration chip to do the computational work. What's presented to the OS appears to be a single drive, which the card then demultiplexes, mirrors, checksums, or whatever into the component channels that go to each drive. Crappy hardware RAID cards do the XOR computations on that processor on the card, better hardware RAID cards do the XOR computations on dedicated silicon controlled by the processor on the card (leaving the processor free to worry about other things). You seem like you generally understand this, so I won't get gory.
Almost all software RAID cards are "RAID optional", meaning that they are designed to be used as RAID, *OR* (often) as plain SATA controllers. Now think about what needs to happen. If you mark the card as being in RAID mode in the BIOS, it *has* to stop appearing to have SATA channels on it, or the OS might do something bad like try to format those "unformatted" disks out there. But now what happens next? The BIOS has to be smart enough to decipher the disks to load up the OS. But that's code running on your PC CPU. So the BIOS knows how to access the disks and read in your OS. Now comes the clever bit. Because modern CPU's are so damn fast, they're pretty much just as capable of doing RAID computations as cheap hardware RAID controllers are, and have been for a decade or more. So now the OS loads a special driver that ... looks at the controller, sees the disks, and turns around and presents a single drive to the OS. This feels no different than loading a special driver that's capable of talking to the processor on a hardware RAID card.
Your OS shouldn't see multiple disks in either case. You may be able to run a "RAID tools" package that shows the actual disks, but how do you tell the difference between using a device driver that is talking to an embedded processor (a hardware RAID card), and talking to a device driver that is actually managing the channels directly (software RAID)?
Anyways, the driver support doesn't exist in FreeBSD, which is no great loss since FreeNAS implements ZFS, which is better.
If you want to look at the current support in FreeBSD for software RAID, see man ataraid(4). It specifically lists NVIDIA MediaShield as a partially supported format, so I'd suggest not going that route.
