Just thought I would share my experience so far with this board as I know that early on there was problems with the IOMMU.
Im running the 2920x with the very latest F12 bios.
I have enabled acs override in bios.
I currently have 1 GPU, 1 HBA card and as I was expected to not be able to pass through one of the onboard NIC's a dual NIC card.
The first boot into Unraid got me some horrible groupings with ALL the NIC's being in the one IOMMU group which was completely pointless.
Setting the PCIe ACS override in Unraid to downstream did sweet fa so ended up setting ACS override to both.
After a reboot all is good again in the land of Aus.
And as the USB 3.1 controller is on its own IOMMU group I "Should" be able to pass that through to my VM, which I will not need for the VM im running but its good to know.
On a side note for this board as im sure someone is going to come across the same issue one day. If you buy this board and you need to do Q Flash plus bios update to get the second gen Threadripper running on it you "MUST" have the new CPU on the board for Q Flash plus to work. No ram or video card needed, but the CPU is a must.