[alan_n]

As a writer I hate these types of apps. As others have pointed out, when you put real popular works in them, they light up like christmas trees. But very few have touched on why.

One thing you learn as a writer is you want varied sentence structure * . Also good prose has a sort of rhythm, both in the words used and the lengths of the sentences (this is also imo why translations rarely compare to the originals). If you make all your sentences the same length it sucks the rhythm out. As for everything else it highlights, they're all just tools. A long word or a passive voice used in the right place can work better than the "correct" alternative. Writing is about knowing which tools to use when. In some ways, writing is very similar to visual design. People don't notice why some writing is good. It's just a bunch of tiny choices that all add up, but also if you use comic sans you're dooming yourself to failure.

What do I actually suggest to writers? Read your story out loud (tts can help to pick out errors as well, but rarely rhythm related errors). If you mentally stumble or backtrack reading a sentence, something is wrong. If you feel like you need to pause but there is no pause, something is wrong. Same with the opposite. Finally, put the story away for a month and try it again. Rinse and repeat until polished.

* Like everything, this also just a tool. You can not use it and still have amazing prose that matches the situation (e.g. one long run-on sentence to emulate a stream of consciousness).