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.