Audio over HDMI makes so much sense, with the connection from PC > AVR > TV, but it just doesn't seem reliable with all hardware and driver configurations. Not just Intel iGPU based HDMI either.
HDMI is far from a perfect solution. It came to us pre-crippled thanks to HDCP, and is plagued by handshaking issues from multiple vendors. And of course the auto lip sync functionality has never worked.
But I think it is the least of a variety of evils. Interfaces that are more capable, like MADI, are not widespread enough to be useful to regular consumers. Reliable interfaces, like S/PDIF don't have the needed capabilities. Best to just try and find a working HDMI implementation.
My experience exactly mirrors jmone's. This is a very unusual situation, but one of Intel's making.
People should stick with the NUC7 if they want it to work, I guess...