Why doesn't my onboard (Fake) RAID work?

Actually, if you do a little searching, you'll find that fake RAID doesn't work well at all for Linux in general, it's not an Ubuntu problem, it's a problem with the fact that Fake RAID is the storage equivalent of a WinModem (if you're familiar with that). The winmodem problem was that the modems were essentially just sound-cards with all the modem stuff offloaded into the driver, and processed by the main CPU, rather than by the modem itself as was the case with real hardware modems.

So the same applies to FakeRAID. You don't actually have RAID, what you have is a storage controller that is mostly brain dead and drivers that handle all the RAID stuff on the main CPU, rather than a dedicated hardware RAID card that has its own processor and firmware to handle all the RAID activities.

For Linux, if you do not use real Hardware RAID, your best, and safest bet, is to use Software RAID, which can be set up in the installer, however, to do so with Ubuntu, for now, you'll need the Alternate installer as Ubiquity (the installer in the standard desktop ISO) does not yet support advanced partitioning and software RAID setup.

SoftwareRAID is a good replacement for FakeRAID and works very well, it's been around in Linux forever and is quite mature.