I came to say the same thing. I use Cloudflare as a DNS provider for my own site as I was impressed by the numbers. It's literally as straightforward as clicking a button to prevent Cloudflare from proxying traffic to use their service only for nameservers.

Pretty sure the captcha can be switched off for enterprise clients (which I presume SO would be).

Regarding caching, SO's caching is extremely aggressive. It's especially problematic when editing answers more than once, because when you click edit you'll be presented with a cached copy of your answer, potentially excluding your latest revisions. So you have to refresh the edit page and then edit the answer.

It might seem like an edge case, and it certainly teaches you to be meticulous about answering lest you have to go through the refresh hell, but it's not what some would designate as 'good UX'.

I am never surprised by SO's reports about how they run their busy website from a mere handful of machines precisely because of the caching they do.