Interesting ....
I've used DLNA and UPnP for years, from a Nokia phone 10 years ago, a couple of radios over the years, to currently having it on an iPad, Android tablet, Jolla phone, XBox360, PS3, Samsung DVD player ...
Only the XBox360 has had protocol issues (They clearly prefer people using something windows).
Only the Android apps are having playback issues (ignoring poor video quality on the XBox360), which seems related to them insisting on punting the playback over to the built-in player.
Media format issues (like supporting flac) is outside the scope of DLNA, since it is the responsibility of the Media Render to support playback of formats. DLNA itself is perfectly capable of handling any audio/video/picture format, but if you chose to use a shite application, that's on you, not the DLNA specs.
Generally I've been pleased with it, it works well for me.
Note: I don't have SAMBA sharing enabled on anything at home, since I don't have anything running Windows. Guess I belong to a superset of "Everyone".