Before I go further, let me say this: the Hemingway App is great.
Any piece of software that can help you make your messages more readable gets my vote. It’s simple and accessible, highlighting your problematic words and sentences according to five colour-coded categories of literary blasphemy.
It suggests a simple word when you try to crowbar in a pretentious one, and it draws your attention to adverbs and the use of the passive voice.
But that doesn’t make it perfect. And it doesn’t mean it’s going to help you churn out perfection every time.