It looks like FreeNAS and TrueNAS is not detecting PCIe drives.
The following was tested in our lab at Supermicro.
SATA was detected by all but not PCIe.
FreeBSD-12.1-RELEASE-amd64 does detect PCIe NVMe.
NVMe U.2: PM1725a (1.6TB) and Intel P4610 (1.6TB)