He mentioned 503 errors and "missed deadlines". I don't think it's costs, they were known beforehand, you wouldn't switch after 6 months because of costs. And I think if you're at that scale, you'd get a counteroffer from Cloudflare if you threaten to leave.

As you can whitelist Tor traffic in Cloudflare, it seems to be down to these 503 errors (edge to origin). But haven't heard that before, so not sure if it's a problem that occurs more often.

For Imgur, I could imagine that purging by cache tag is just too restrictive at Cloudflare (the limit is very low, even for Enterprise clients). Fastly doesn't have a limit there, they encourage you to cache everything and purge where needed. Makes it much easier to cache APIs and HTML pages.