Earlier quoted context omitted.
The only thing the end user has is the difference between http:// and https:// . Cloudflare undermines that entirely. How can a user possibly ever know whether it's safe to enter their credit card number or medical information in a web form, in a world where CloudFlare "Flexible SSL" exists?
If a user thinks the presence of "https" means it's safe to enter credit card details or medical information, that's already a huge problem. Yes, "https" should be a prerequisite to entering sensitive information, but that's only part of it; the other part is whether you actually trust the server you're sending that information to. The server could be using ironclad encryption across the whole connection, but that do…
This is true, but conversely there is no legitimate use case for Flexible SSL. Having a datastore like Redis or MongoDB that by default listens insecurely on any address is almost as bad, and such things often compromise the security of a site if it e.g. sends your data across the internet to one of those, but at least there's a more-or-less legitimate use case for that default if it's used on a secured network - it's at least possible that someone using that default isn't deceiving their users. Whereas anyone using Flexible SSL is necessarily deceiving their users (I mean you can argue users might genuinely think "I don't trust my local cafe operator but I do trust the completely public, unsecured internet", but I don't think that's a coherent position for anyone to take).