I suspect many of the responses will be overly critical in a way that is borderline unfair.

This is an AI informed by the opinions of multiple scholarly experts. I'm sure they'll keep smashing "train" and "generate" and adding new inputs until it meets some standard of sounding like authentic music. The public already accepts several completions of deceased composors' works. People pay to hear Simon Rattle's fourth movement of Bruckner's Ninth. Everyone plays the same version of Fantaisie Impromptu despite instructions to never posthumously publish Chopin's manuscripts. There's a Youtube completion of Morning Glories that receives great praise despite the missing sections lacking faithfulness to Scott Joplin's technique and style. If David Cope's algorithmic machine had announced it had written the missing sections, he'd be burned at the stake by his Youtube audience.

Against the criticism that this sounds like existing Beethoven music: I'm relieved it doesn't sound like new Stravinsky. New Metallica often sounds like bits of old Metallica except strange and different and sometimes confusing.

It sounds decent. There are probably elements of "new and different" that are missing, but there are repeated themes with variations, interplay between minor and major keys, and similar note/rhythm structures to historical Beethoven---maybe not as much dynamic variation, tutti rests, and solo/soli melodies as the Ninth. I admittedly don't know much about the full catalogue of Beethoven and expect many of the critics here also mostly know the Ninth.

My personal wish for gauging this work (once it's complete) would be more quantitative: a "classical music earworm neural net" plus a "composer uniqueness rating".

The first would basically max out on La donna e mobile or Messiah. These are the melodies that would be instantly learnable and widely recognizable. If each Beethoven composition rates higher than the last, we can expect the Tenth to be the Gold Standard of earworms. This could be your alarm clock tone and still never get annoying. Otherwise, we'd need to accept that the posted version isn't absolutely epic but still more worthy of release as a "what if" instead of doomed to history as a bunch of lost, incomplete sketches and musical riffs of a angry, sick, deaf lunatic. A best guess, if you will.

A uniqueness AI should be able to say Pictures at an Exhibition and Night On Bald Mountain came from the same pen but are not built with the same bricks. That is, the composer and style are the same but the themes and melodies are unique even though they share the same twelve notes, twenty lengths, eight dynamic markings, etc. Until you can show that Bach never ever repeated a lick from one chorale to the next (as well as what constitutes the length of a lick... seven notes? ten seconds? two measures?), it's unfair to accuse this composition of borrowing from other works. If the Proposed Tenth scores the same uniqueness as any other Beethoven work when removed from its training set, there would be no leg for the argument that this is just a derivative work. We are all derivative works making more derivative works.

An extra wish would be to know Beethoven's published versions varied from the early sketches. (Someone already commented this same thought.) From what I recall, Bruckner's Ninth underwent multiple revisions before he finally croaked at 80 percent complete. His first sketches were reportedly not very playable but were fine-tuned by him as well as a few well-known contemporaries. If Beethoven always kept adding to sketches but never really changing the line, we can assume everything in the sketches here would belong in the symphony. Otherwise, I hope the research team is figuring out a way to guess what all the changes would have been.

In short, don't knock the arrangement just because you know it was informed by an algorithm. After all, what are genius composers if not masters of taking only existing music and established rules and bending them in new ways---in other words, living algorithms?