I actually thought I was being quite generous.
As far as I can tell the WiiM UPnP implementation is essentially just the Linkplay hardware coupled with the Linkplay reference app, which Linkplay have been producing for years and years. If you search for Linkplay apps you'll see them scattered over the various app stores e.g.
https://play.google.com/store/apps/details?id=com.wifiaudio&hl=en_GB&gl=US.
Given they've been churning these out for years you'd assume they'd get the basics of a UPnP renderer right, but in the few weeks of owning one I've identified a couple of bugs to do with their URL handling meaning they won't work with my server (MinimServer) or others like Kodi, LibreELEC e.t.c.
In addition to that, when using it as a UPnP renderer from a third-party app the WiiM only manages to play tracks that are ~>30 seconds gaplessly, whereas Sonos and Volumio (the only ones I have on hand at the moment) have no problem with tracks that are only 5 seconds. Whilst in the real world it might not be a problem it just highlights that the implementation isn't as robust as it could be.
Interestingly one of the URL/naming bugs isn't a problem if you're using the WiiM Home app nor is the gapless test, so I've no idea what's going on behind the scenes!
In terms of the in-app UPnP browsing, the media server sends the track metadata to the control point using xml in the order that it's to be presented, all the app needs to do is to parse the xml and display it. Only in the last couple of weeks did they implement the correct track ordering for multi-disc albums (at the expense of a bug), and the fact we still don't even have track number in the track listing is hard to excuse. As I said earlier I hold Linkplay to account for most of this, but the fact WiiM haven't moved this on in over a year is really disappointing.