I have a Denon AVR-3808 audio receiver. This appears as a Network Audio device in Settings/Connected devices, but does NOT appear on the Action Centre Connect page, so I can't connect to it.
This means that I can't use a Modern UI app (e.g. Music or Music Preview) to "Cast" my audio to the Denon. I can do this in Windows 7, 8 and 8.1, but not Windows 10, it seems.
The Denon is a DLNA DMR class of device for audio.
Interestingly enough, I can use the traditional Windows Media Player to "Cast" to the Denon; and the function, which was called "Play to" in previous versions of Windows is now relabelled as "Cast to" in the WMP menu.