The free version of cloudflare is essentially a "fair-use" offering, meaning that it is free until they decide its no longer free for you at their sole discretion [0]. Their terms of service prohibits distribution of any "disproportionate percentage" of non-HTML content [1]. That includes most of what you would want a CDN for: video, images, audio, binaries.

They have exercised this discretion repeatedly for significant bandwidth users, usually in the form of "You need to upgrade to the enterprise plan or we will terminate services for your site." One of my sites got the enterprise "offer" after serving single digit TB in a month. Running on a real CDN from the start would have been cheaper than inevitably getting extorted to a large <contact sales> price for <contract term> or terminated with little to no time to migrate things.

[0] Section 2.6 https://www.cloudflare.com/terms/ [1] Section 2.8 https://www.cloudflare.com/terms/