The downside of hosting things all in the same domain is that cookies are shared between them, so a vulnerability in one site (e.g. XSS) leads to compromise of all sites. Choosing different domains means they are sandboxed and safe from each other.

Any domain name could be used to host porn. But not any domain name can get linked from a cloudflare blog. I think the fact that it's linked from cloudflare's blog should indicate that it's fine.