A lot of the human-generated public domain translations are public domain because they are pre-1920s and are pretty hard to read.

The translation of Les Misérables that is the "official" edition they sell at the gift stores if you see the musical is, to me, almost unreadable compared to modern 21st century translations.

I would think there are an enormous amount of books out there that would be improved by a modern computer translation, even with its caveats.