Volume control is not working correctly from Client: Changes in volume from the Client are not updating the volume 'dial' in the GUI (did not work in 1.55 but would be nice).
Auremo shows changes made to volume by MPod/Pa D but Volumio does not show changes on its volume slider (in 1.55 changes made in the Volumio GUI were displayed on Auremo). Clients seem to control volume in a linear manner disregarding the 'Volume Curve Mode'setting from within playback. Clients have no control in either 'Linear' or 'Natural' mode.
I can fully understand that this is not a priority (as stated in the comments, you invite for some contributions on that matter), but in that case, I would suggest removing MPD from the core, have it as a plugin and remove it from the features that you advertise in the documents, thus reducing the hopes in that a potential user can have of that functionality (or explain that it's an either/or type of functionality).
From what I gather Volumio does actually still use MPD (correct me if I'm wrong), it's just that the web-interface does its own thing with regards to playlists etc.
This was a rather dirty approach to adding music to my library, and I cannot recommend it to anyone (Especially since it involves using GUI programs…).