Here's some anecdotes I dug up: https://news.ycombinator.com/item?id=38960189
Put another way, if Cloudflare really had free unlimited CDN egress then every ultra-bandwidth-intensive service like Imgur or Steam would use them, but they rarely do, because at their scale they get shunted onto the secret real pricing that often ends up being more expensive than something like Fastly or Akamai. Those competitors would be out of business if CF were really as cheap as they want you to think they are.
The point where it stops being free seems to depend on a few factors, obviously how much data you're moving is one, but also the type of data (1GB of images or other binary data is considered more harshly than 1GB of HTML/JS/CSS) and where the data is served to (1GB of data served to Australia or New Zealand is considered much more harshly than 1GB to EU/NA). And how much the salesperson assigned to your account thinks they can shake you down for, of course.
Their terms specifically address video/images:
> Cloudflare’s content delivery network (the “CDN”) Service can be used to cache and serve web pages and websites. Unless you are an Enterprise customer, Cloudflare offers specific Paid Services (e.g., the Developer Platform, Images, and Stream) that you must use in order to serve video and other large files via the CDN. Cloudflare reserves the right to disable or limit your access to or use of the CDN, or to limit your End Users’ access to certain of your resources through the CDN, if you use or are suspected of using the CDN without such Paid Services to serve video or a disproportionate percentage of pictures, audio files, or other large files. We will use reasonable efforts to provide you with notice of such action.
https://www.cloudflare.com/service-specific-terms-applicatio...
I was going to say that it's odd, then, that reddit doesn't serve all the posts' json via a free account at cloudflare and save a ton of money, but maybe actually it's just peanuts on the total costs? So cloudflare is basically only happy to host the peanuts for you to get you on their platform, but once you want to serve things where CDNs (and especially "free" bandwidth) really help, it stops being allowed?