1. Does it properly stop the playback when headphones are disconnected?
2. Is there a usable lock-screen widget?
3. Does it auto-rewind after a pause? It's best when the pause duration is taken into account: it shouldn't rewind more than a couple of seconds if you pause for a second, but a whole minute may be better if you left it off yesterday.
4. Does it handle well situations when the server is not accessible? Can you just pre-download a couple of audio-books for your 10 h long flight? I mean, honestly, streaming is never desirable, the only point of a self-hosted server is that you don't have to download and delete your collection manually. It's a tricky problem when the playlist is highly variable (as with music), but for an audio-book player should be a non issue: just pre-fetch a whole book (maybe manually), storage space is a lesser issue than mobile internet.