Interesting predicament.  I am running all 6 vega 56 at GPU P7 1408 MHz 1000 mV and  memory P3 1100 MHz 950mV.  Where it seems to be fairly stable.  I did get 1 hang in GPU0 which caused a restart.  However, when I undervolt the cards down to GPU 841 mV and memory 805 mV GPU0 that hangs in openCL or something like that.  It is always GPU0 and that confuses the hell out of me as these are all Sapphire RX Vega 56 and all have samsung memory.  The only difference is that GPU0 is in the actual PCIE slot on the motherboard and the others are all in risers.  It does have a monitor connected to it right now but that will be replaced with an HDMI dongle shortly.  Thoughts anyone?