Varnish also supports plugins for extreme flexibility. For example, I wrote a plugin for our Varnish install which performs HMAC validation of a specific signed

Varnish also supports plugins for extreme flexibility. For example, I wrote a plugin for our Varnish install which performs HMAC validation of a specific signed cookie and then sets a header which is used downstream in the caching rules. Varnish is mature, powerful, and fast as hell. It would take a lot of work to reach a point where I'd swap it out for something else.