Thanks for the insight! I've tried a few DLNA apps now and indeed, that seems to be the way to go with the Wiim for music stored on the local network.I have a Synology NAS. I use their DS Audio app on my phone and it works as expected with Wiim (using DLNA protocol): I can search, it has random options, playlists, etc.. everything one would like to have. Again, that is on the DS Audio app, not on the Wiim app. I think it's up to the app you're using with your NAS to stream using DLNA to your DLNA receiver: Wiim in this case. The Wiim app is good for other things. I personally use Wiim app only to stream radio, when I am not using AirPlay, as Spotify&Tidal have their own options (Connect) in their apps.
Those same apps also support streaming to the Raspberry Pi so I'm probably going to return the Wiim. Until Moode's UI is fixed, I can experiment a little more with those DLNA client apps.
I also noticed that with playback of high resolution (24/192) files, the Wiim takes some time to start playing - especially longer compared to my Raspberry Pi (with ethernet cable). Otherwise, it's a very solid little device for those who want to buy their first streamer.
It seems like for the best experience with local music, a Bluesound Node is probably the way to go - with a much better app? But all of this won't matter that much to me anymore when Spotify launches their Hi-Fi tier.