Power Consumption
Performance and pricing aside, power consumption is by far Vega's biggest hurdle. In all honesty, performance isn't bad and it might even improve quite a bit over time. Moreover, once pricing settles down I'm sure that will be fine as well. What won't change is the power consumption. Sure you can undervolt and I'll look at that in the future but for now this is what most Vega owners will be faced with.
Despite using a bit more power than the GTX 1080, Vega 56 really isn't that bad. Vega 64 on the other hand is a serious power pig. The already hungry air-cooled version makes the GTX 1080 Ti look like it's on a diet, while the liquid cooled Vega 64 behaved like a bear at a buffet.
Seriously though, pushing total system consumption 24% higher than the GTX 1080 Ti for on average 18% less performance is an issue. Not everyone cares as much as me about power consumption but it does come at a cost.
In its current reference card form, Vega is a 'hangry' graphics card that will take its problems out on your power supply.