Adobe software sucks as usual. Nothing new there.In premier pro Vega can’t be used to hardware accelerate encoding but Intel can with quick sync. This is often the case, the Intel iGPU is a lot weaker but it does support more software out there. Reviewers should cover this and focus less on gaming as I agree with you that people don’t buy these things for aaa gaming.
Another question is Why it does not support generic GPU acceleration. Simple answer: it sucks.
Another POV is that there are ton's of software that do not support Quicksync. You can always cherry pick some examples but they won't change big picture.