https://old.reddit.com/r/programming/comments/1d14rb7/cloudflare_took_down_our_website_after_trying_to/ is the most recent Cloudflare drama. They've been known to fuck customers before but I can't really find specific examples. Obviously their protection can also be overzealous and block legitimate traffic too, which pisses off users as well.
Beyond that there's many more philosophical reasons to hate Cloudflare - they're a highly centralised point of failure and like in the story linked above could at any time "alter the deal", so to speak. As an advocate for the free and open internet I wouldn't consider them a force for good any more than Google, Facebook or Amazon.
They're also hated for blocking privacy tools like Tor and blocking scraping, which does suck, but if cloudfail doesn't work anymore you can still always search SHODAN for website title/headers to see if the LB is accessible directly via the internet. DNS management at medium sized corpos is usually a clusterfuck so it's definitely a non-zero chance.