I have a Roon Nucleus One attached to a Marantz AV10 through the media player hdmi input. When I play music through this, album art never shows. I know that the Nucleus One HDMI connection is for audio, not Roon’s album-art video interface. Is the only way to do this is to not play directly through the HDMI Roon Nucleus One interface, but rather use the AV10 Roon Ready HEOS option? What is the limitation in sound quality running through the AV10 Roon Ready interface rather than through HDMI and the Nucleus One. Thanks.
The big limitation is that multichannel audio can only be handled via the HDMI interface. Using RAAT over ethernet to an AV10 will only handle stereo audio. Note that this limitation is not caused by RAAT, but by the hardware/software in the AV10.
I believe that other aspects (sample rate/bit depth) are the same for both routes, but I’m not sure.
I have tried a multichannel SACD using the HDMI and even though I select multichannel on the av10, I never seem to hear my rear surrounds playing music. It happened once or twice but then never again. I am not sure why that is. Roon identifies it as 5.1 but does not seem to play that way. I only have 2 multichannel SACD’s so it is not that big of a deal. So then the question is I can play through the roon ready AV10 rather than through the HDMI. The question is if there is a loss or limitation doing it that way other than the lack of multichannel.
Well, the simplest way to find out is to try it for yourself. You can see what the settings are in the device setup screens for both the HDMI connection and the Roon Ready connection in Roon’s 
> Audio devices screen. And the Signal Path for these two Zones will show exactly what is happening in real time.