First let me say: Wow, I don't want a line editor's job. It's important, and apparently in-demand, but it is not my kind of "fun".
Of the changes proposed, I accepted a couple hundred, made alternate changes to three, and rejected five. I figure that's a pretty good indicator that the service was not only valuable in detecting mistakes, but also in correctly fixing them.
The two most common things fixed by this edit were "putting a comma before an 'and' in a list" and "not spelling out numbers". Both of which could be automated, but there are SO MANY fiddly exceptions that you really do have to check each one.
There was a lot of other good stuff though. Places where my sentences were overly complex, and they tweaked them a bit, a couple of places where I'd used "alternate spelling 1" and they preferred a different spelling. Although technically not a line-editor's job, they even detected an unsupported statistical claim, and flagged it. Overall, I got a lot more than I'd expected, but I'd also paid more than I expected, so I figure I got what I paid for, and that's the important bit for me: I really do seem to get what I pay for.