I think people are reading a bit too much into the name, and into the feedback the app gives. I don't think it's "get rid of all the pastels and your writing will be like Heminway's", nor do I even think "get rid of
all the warnings" is what it's trying to do.
It is useful for a writer to throw some text at it and see what you can learn. More feedback is almost always better for writers. The trick, is always, is having the judgement to incorporate intelligently.
For example, I stumbled onto a book about procedural content generation in games[1]. As a writer, game programmer, and dedicated fan of roguelikes, if this book were any farther up my alley, it would be banging against the back fence.
But, ugh, when I tried to read it, I just gave up after a few paragraphs. It's not gibberish, but it's almost physically painful to wring the actual information out of it.
And, indeed, when I throw some of those paragraphs at this app, I see:
Paragraphs: 1
Sentences: 31
Words: 833
Characters: 4196
11 of 31 sentences are hard to read.
11 of 31 sentences are very hard to read.
10 adverbs. Aim for 0 or less.
10 words or phrases can be simpler.
13 uses of passive voice. Aim for 6 or less.
If the authors took a bit of advice from this app, they'd end up with a better book. That sounds like a win to me.
[1]: http://pcgbook.com/