I work in the CDN industry and am also heavily involved in the FreeBSD community. I find Varnish to be kind of a tragic curiosity in that is has nice fit and finish but is pretty unsuitable for high scale workloads due to the influence of its creator phk@. Ironically on the OS he should know like the back of his hand, sched_ule(4) has fairly expensive depth searches to ensure process fairness. Doing a thread per connection is about the worst conceivable design for web acceleration. It is probably ok for low connection count high bandwidth (like chunked video streaming), but you're still going to be dropping performance on the floor vs other I/O models as you get into the 1000s of connections, and as Netflix has publicly shown that is modern reality on FreeBSD with 100gbit networking.

My understanding is that fastly has rewritten the I/O model upon more industry standard event polling worker threads. They also reworked a lot of the memory model using https://github.com/fastly/uslab. None of that I/O work is upstream and it's unlikely it ever will be because of the phk@ influence and perhaps fastly sees it as IP. So be prepared to do some deep systems work if you need high scalability out of Varnish.

Apache Traffic Server is the most ready "out of the box" caching software for rolling a CDN that I've seen. If you're going to do custom development, nginx is a good I/O and HTTP model but be prepared to roll a complete cache storage model from scratch.