My observations on the X470D4U:
CPU: Ryzen 5 3600
Cooler: Dynatron A24
RAM: (2) Samsung M391A2K43BB1-CRC 16GB DDR4-2400 CL17 ECC UDIMM
Slot 6: Mellanox ConnectX-3
Slot 5: empty
Slot 4: LSI SAS2308
I was able to overclock the RAM to 3200 CL 18 at stock voltage and 3400 was mostly stable. I expect I'll be able to hit 3600 at a higher voltage. There is currently no EDAC support for Zen2 in the linux kernel (coming in 5.4) so I didn't want to mess with the RAM too much without getting feedback from EDAC.
Unless I'm missing something, the max you can push the RAM voltage is 1.20V +/- 124mV so 1.324V is as high as it'll go. I'm surprised you can't set it to 1.35V.
In order to use both the x16 (physical) slots you must configure the 16 lanes as 2x8 in the BIOS. I'm used to that being automatic on other boards. The options aren't terribly flexible: 1x16, 2x8, 4x4. It would be nice if you could able to do 1x8 + 2x4 so you could have a single x8 card plus a dual NVMe 2x4 card in the other slot.
I'm getting 512 byte PCIe packet sizes for the Mellanox and LSI HBA cards, which is nice, but the max packet size for devices hanging off the X470 SB is stuck at 256 bytes. Fortunately, the packet overhead is less painful with PCIe3 than it was with PCIe2.
I ran into a strange compatibility issue with my ASUS VN248H-P monitor. When it was plugged in the BMC would crash regularly during reboots and would take several minutes to recover. The BMC has been perfectly stable since I unplugged the monitor. Strange.
Fan connector FAN2 only works correctly in smart mode (ie temperature following mode). When set to a fixed speed the fan will only spin at the minimum. The same fan connected to either FAN3 or FAN4 works as expected when set to a fixed speed (eg full on). Perhaps FAN2 is intended for coolers with dual fans. I did not test FAN5 or FAN6.
My 2U chassis doesn't have enough airflow to handle the 65W Ryzen 5 3600. Under mprime load, the CPU temp hits 90-93C (thermal limit) and and the all core boost drops from 4GHz-ish to 3.8GHz. That's still better than the base clock which is 3.6GHz. Perhaps fans optimized for high dynamic pressure would help but ultimately I think my chassis is just not airflow friendly.