Kodi does not handle audiobooks/podcast at all, lacks playlist edit API and lack transcoding.
Currently there’s no perfect API for Audio that covers everything 
Not talking about some features some servers currently have and may want to expose via this API too.
A new open standard could improve the whole ecosystem but I doubt that this will happen. Plex will keep it’s private same for Kodi Emby or Jellyfin.
Subsonic API is the standard that is currently used by more servers than just it’s originator.