I'll add my Ryzen experience so far. Just installed it, I didn't have a 1x Gen 3 slot so I gambled and used a 1x Gen 2. Been playing Pavlov VR all day no problems, everything appears to be working flawlessly.
Specs:
Ryzen 1800x OC'd 4.0ghz
16gb DDR4-3200 running at 2933
RX Vega 64 (-50mV core volt, stock core clock, 1060mhz mem, +50pow limit)
AsRock X370 Killer SLI/ac
Didn't change any PCIe BIOS settings (just the OC/overvolt). Camera was already disabled. All the firmware/drivers/BIOS was updated prior to testing. Like I said its running in a 1x Gen2 slot without an issue.