It's a good tool. However, last I checked, it was not possible to run it in a one-shot stateless fashion, like, passing it a list of music files so it auto-fetches album art, lyrics and updates the very same input files.
I'm still not totally sure I got all the certainty thresholds right, I still sometimes get unexpected behavior when using Beets this way, so I agree that the non-interactive experience is lacking.