It's no secret that those products are free* with a big asterisk, if you hit some unspecified combination of total bandwidth usage, attracting DDoSes, serving the wrong types of files or moving too much data in regions where bandwidth is expensive then you get an email from sales asking you to pay up or leave. Even if you're on a paid plan, beyond a certain (also unspecified) amount of usage they will ask you to pay more, until you end up on an Enterprise plan where you pay-per-GB just like every other CDN.
They are currently quite generous with how much you can use for free or cheap, but the limits being so vague means they are subject to change without any transparency, potentially pulling the rug on you at any time. That's what I mean by suspiciously cheap.
