Readability formulas have even more limitations than Klare discusses
Janice C. Redish · ACM Journal of Computer Documentation · 2000
A literature review reveals many technical weaknesses of readability formulas (when compared to direct usability testing with typical readers): they were developed for children s school books, not adult technical documentation;they ignore between-reader differences and the effects of content, layout, and retrieval aids on text usefulness; they emphasize countable features at the expense of more subtle contributors to text comprehension.