UPnP/DLNA seems to be a bit of a disaster protocol wise. As you say the codec support is "eh whatever" which means whether it works or not depends on too many random factors. It's also a massively over-complicated design-by-committee relic.
There also isn't a good open source DLNA server (presumably because of the above issues).
That's actually pretty fair. I was ready to jump in and say that miniDLNA is such a server but having looking I see it's actually been unmaintained for 10+ years! It has worked pretty well for me but I think I must have just gotten everything in the right formats years ago so I don't notice the issues I see reported now (I used to just restart it to avoid the notify issues).
Presumably you still need some kind of media server for the fcast receiver to stream from when playing local media, I assumed that would be dlna based but what is the actual idea here?
There also isn't a good open source DLNA server (presumably because of the above issues).