FYI, Soulnight at AVS Forum started a thread about the Envy. I just saw it today and found some interesting tidbits about people's expectations for this thing:
madVR ENVY: Anticipation thread!
One limitation not mentioned versus a HTPC is that madVR doesn't have access to the decoded frames. If you used a Nvidia Shield as the source, for example, you would still have to live with the Shield's flawed video output. Devices like the Shield do let you choose different output resolutions for different content, and there are obvious advantages such as accessing a few 4K HDR movies that will never be ported to UHD Blu-ray or 4K HDR Netflix and Amazon TV content.