Vega 64 can in theory do 25 TFLOPs half precision.

But as you say there's a large price difference too.

For a market segment that needs 1-8 GPU rigs for ML on a low budget AMD could kill it if they invested in software support and kernel optimisation.

For servers and large scale training, unless AMD has some ML specialised cores in the pipeline, Nvidia Volta and Google TPUs have a serious lead.