I recognized some issues with the Deezer implementation.
1. It seems like the authorization gets lost after some time. Then there's random content within my library and my flow. Logging out and in again solves the issue temporarily which is quite annoying.
2. When playing my flow the data and cover of the upcoming track are shown within the player.
3. When playing my flow or an artist radio the FLAC symbol is not shown so I guess it's not flac.
4. When I subscribe to a playlist on Deezer this playlist is not available in my library. I know other players that have seperate sections for own and subscribed playlists. The latter seems to be missing in WiiM's implementation.
Are these known issues? Can I hope for a fix?
Addition: 3 also applies to mixes (Mixes -> Choose one).