Although, as ever on these topics, it's probably worth finding out what professionals think of these tools (spoiler: not much although this is from 2014 and it's possible they've improved it since.)

https://languagelog.ldc.upenn.edu/nll/?p=10416 "Filed by Mark Liberman under Prescriptivist poppycock"

edit: Just checked and it still rates "Silly" x 75 as "Good". "Hemingway" x 30 rates as "Post-graduate" but otherwise good. Yeah, it's still broken.

Thanks for reminding me about this! I had seen it years ago, but forgot about it till now.