Honestly, for me it's kinda minimal, simple and ergonomic UI is the point. It has some more sophisticated features, but they don't distract me, nor I actually need them. For instance:
- bookmarks (I tried to use them, but it's too much work, if I really need to take a note, I better do it in written form)
- UI to write down characters (same: I used it a couple of times, just because it's there, but it's not worth it, use normal note-taking app for note-taking)
- equalizer (I'm sure somebody uses it, but I never truly needed it for audio-books)
And many more.
What actually matters are small details:
- Granular playback speed control. ABS seems to have it done right too, but some players have like 1-1.25-1.5-..., which is not granular enough to get normal conversational talking speed out of somebody way too slow.
- It is possible to enable ID3, but it doesn't force it either.
- It recognizes chapters in whatever I have. m4b, mp3, cue, many files in a single folder, many nested folders, whatever. If it gets things wrong, it is possible to merge books in the library. I guess, the worst case is if I am overzealous with chapters (like marking chapters in a youtube podcastwhich has a mark like every 2 minutes), but then again, it only means "stop at the end of the chapter" becomes not feasible, but otherwise it's fine.
- I guess, every audio-book player does, but still: it does remember positions in multiple books, so I can switch between lectures and fictions and get back to where I stopped.
- It takes into account the current playback speed when estimating how much time is left.
- It is possible to remove buttons you don't need, it's possible to choose an action at the end of the book, it's possible to choose a lot, but defaults are really just fine for me.
- I never used it, but thinking about your question I just found out it has "Skip start/end" functionality. This is just great, I'm excited to try it out ASAP.
Anyway, I probably forget some small details, so you better take a look yourself. It's not perfect, but good enough to study when building something similar. I can also suggest 1 thing that is not in SABP, and, again, it's important to not overload the UI, but I sometimes wished it was there. In SABP it's only possible to either stop after X minutes (with sleep-suggestive volume fade), or at the end of a chapter (without fade). Sometimes I wish I could tell it to stop at the end of a chapter approximately after X minutes (like, if it has a chapter every 7-15 minutes, and you are planning to listen for about 30-40 minutes more, but you don't want it to fade volume an to stop in the middle of a chapter overall). So it's either "approx X minutes" or simply "N more chapters". Neither is possible in SABP.
Also, I don't know it I actually want it, but it seemed nice listening to podcats in NewPipe (SABP doesn't have it). There is an option to skip silence in NewPipe. I didn't actually use it too much, and the last time was a while ago (I pretty much stopped to listen podcasts on it), but I seem to recall it worked nicely.