Having worked on readability algorithms before I think this is a well-designed take on improving the usefulness of readability scores. Knowing that your text scores high or low in terms of readability only gets you so far.
Hemingway additionally uses colour-coding to make suggestions such as:
- split sentences
- use fewer adverbs
- use simpler phrases
- use less passive voice
Colour-coding sections of a text that merit improvement both calls for action and helps with recognizing what exactly should be reviewed and in which way a particular arrangement of sentences or better choice of words can help with understanding a text more easily.