My experience with Hemingway in the past has been that it's a set of general heuristics which prevent bad writing. I found it quite helpful to proofread. It's not a replacement for a human editor, but it's a step up from traditional autocorrect.
At the end of the day, it is still up to the writer which suggestions they keep. Anyone who blindly accepts suggestions because the tool recommended them probably wasn't in the correct mindset to write well anyways.
As others have pointed out, the application of such a tool for the prevention of bad writing will, at best, produce writing that is merely mediocre instead. One but hopes that the common reader of HN aspires to produce writing of a quality beyond the mediocre, unless she is usually assigned with the most trite and dreary of tasks.
Think of it as an attention getter. Rather than saying "you need to change this," it's a way to say "Make sure your usage of this construct is thoughtful and intentional instead of accidental."
To that extent it seems like it could be useful in certain contexts.
At the end of the day, it is still up to the writer which suggestions they keep. Anyone who blindly accepts suggestions because the tool recommended them probably wasn't in the correct mindset to write well anyways.