Quantity 6 AMD RX Vega 56 confirmed working on ASRock X370 Killer SLI/ac AM4 AMD Promontory X370.  More trouble than other motherboards I listed before but nothing terrible.  Had to DDU/reinstall drivers a few times before it took with all 6 GPUs on my base image SSD.  Worked once I "express" installed the AMD driver back on top of itself again after an initial DDU and re-install.  I've frequently had troubles with Asrock boards in the past (bad voltage regulators in old ones) but don't want to assume it more than a fluke.  We all know how AMD drivers can be.  Working fine for a couple days now.  Nice to have wifi onboard if needed and the price is right for the features.