Reading around online, it seems egress is free until Cloudflare asks for big bucks or threatens to terminate your service. It's probably fine for hobbyists or small web sites. But as a business, I would never trust something that is "free". There has to be a catch somewhere.

A business contact of mine asked Cloudflare for a quote for setting up a CDN for 7B requests per month. They dragged him through five sales calls to eventually deliver an offer with request costs 30% above CloudFront's public pricing. He said the costs per GB were ok though.

The cheapest reliable CDN I've found is bunny.net. Unlike OP, who is selling a book about Cloudflare, I have no conflict of interest by recommending Bunny, other than being a customer. I serve around 100 TB per month through them, at $0.005/GB. You can put it in front of a bucket to have cheap (globally distributed) egress with all the benefits of your favorite object storage provider. In my case, the buckets lie on Linode/Akamai. Bunny uses CDN77's infrastructure, which I have also heard good things about.