I am running iOS and therefore are only able to stream from ARC to Airplay. Android users OTOH seem to be able to only stream to Google Cast.
Being able to stream to both protocols would remove the last obstacle to use ARC in places which are not equipped with high-end devices or streamers (i. e. visiting relatives or holiday houses).
Apart from that, ARC is a joy to use, cheers… 
Best regards
Casting to Airplay or Chromecast from a mobile device are a feature of the respective operating systems, not Roon ARC.
Well, but then again, how is it possible to cast to Google devices from Plex/Plexamp/Spotify/Netflix and many other apps, but not from ARC? Am I missing something?
Maybe. You’re listing servers, which could also include Roon, not iOS or Android apps. Does the iOS version of Qobuz or TIDAL support Chromecast?
I don‘t know, because I use neither of them. And I was listing Apps, as - with the exemption of Plex - none of these Services run on a Server at my home. Of course, they run on some server, but so does Roon (e. g. Plex), and Roon can do Google Cast anyway…
You can also add Amazon Prime, Youtube (Music) and many many more. I doubt, any of these have anything from Roon implemented…