I spent a while trying to make this work and I'm sure others have too so I'm posting this to have both question and answer available.
- ASRock X399M motherboard
- Threadripper 2
- 3x 4TB SATA3 HDD configured RAID 0
- 2x assorted non-RAID disks
The AMD RAID (on-board SATA and NVMe RAID chip) drivers are not installed in Ubuntu 18.04, so the disks show up by themselves but only when AHCI is enabled (the default AHCI driver is used). Enabling RAID mode shows nothing of course.
The driver is not available through AMD support for X399M but rather an AM4 chipset like X370. As of now, the driver can be obtained here but if link is broken, visit amd.com support section for driver download.
After download, attempting to install this AMD RAID driver manually from source failed immediately. Ubuntu 18.04 comes with kernel 4.15 with which this driver is incompatible.
Normally I would say "Someone please point me in the right direction" here but I happened to find the answer just now and will post below.