Hemingway the writer actually wrote long sentences and they were actually important in his writing.
Passive is also important in good writing.
You can't use machine metrics to force "good writing" you can only enforce mediocrity and the following some random rules "because the rules have to be followed."
Likewise, I as a writer of the software would absolutely hate to run some program to tell me "this function has more than 10 lines" or whatever. If I wrote 500 lines function it doesn't mean it shouldn't be that long: there are examples where exactly such functions are still necessary and good. Such automatic evaluations are for managers who probably don't understand what they enforce. Pointy-haired bosses, if you will.
So I see HemingwayApp as the pointy-haired-editor app.
(Edit: Improving the text based on the human input, thanks Agathos!)