I had the same problem with an Asus motherboard and a rockrtRAID card. The problem was that I wanted to boot from a RAID array created with the AMD raid controller and if I installed the card in the computer, then it would not see tha AMD RAID and could not proceed to boot. The same setup was working right with a Biostar motherboard. So yes, Asus BIOS has a bug.
But I finally made it working by flashing the rocketRAID card with the option of not bootable. That prevented the BIOS to mount it as a bootable device, allowing me to normally boot fom the array.
So, the problem seems that you cannot have 2 RAID controller set as bootable at the same time. The lack of cache memory seem a plausible explanation for that, as it happen only with Asus motherboad (at least in my case).
There is ways to fix that behavior.
1) If you plan to boot from the rocketRAID card, just set the AMD controller to IDE so BIOS won't have to mount extra stuff and you'll be able to boot. Not mounting the AMD RAID/AHCI bios will leave room for the rocketRAID bios.
2) If you want to boot from the chipset controller, flash the rocketRAID card (mine was an old 2300 PCI-e controller, don't knw if the new one have this option) and disable the boot ability of the card.
3)Buy another not Asus motheboard...