I think people seriously overestimate how useful CDNs are for static assets. Even if the page you visit happens to call for the same version of a resource you might have cached, there is no guarantee it is requested from the same CDN. And all these hostname lookups slow things down too. I frequently see a site request assets from multiple different CDNs when those assets are all available from a single CDN. The designer clearly copied a snippet of HTML without regard for the extra lookups when they could have found all the necessary assets on a single CDN.
I tried finding reliable data for cache hit/miss rates for popular CDNs but was unsuccessful. Is there a site out there tracking this that designers could refer to? It would be good to know if simply concatenating all your CSS (for example, or JS) into one file and serving it from Cloudfront would beat scattering it across several providers and hoping the majority of visitors had cached copies. I suspect the single file would perform better but I have no reliable data to support this.