Volta's memory bus is 3072 vs 2048 bit width of Vega. It is 1.5x more bandwidth if using same freq. moreover I think NVIDIA is doing better with timings tuning
Im sure the timings differ and that could boost performance, but considering the Vega 64 can do 43mh/s maxed out, just tighter timings cant be pushing a 90% increase in peak performance, can they? (bear in mind these are stock timings, so they wouldn't be as aggressive as the STILT's straps).
Memory bus can be an additive factor, but then the GTX 1060 run a 192bit memory bus and outperform the 256bit RX 470/480/570/580 cards at same clocks (without bios mods ofcource).
Something tells me the Vega 56/64's are crapping out due to some other reason ...