This is a shame and would have made plesk very attractive.

you can install varnish along side plesk.. you need to manually allocate the IP to nginx using the Listen command and reserver an IP for varnish. It should be simple to automate this in plesk

2 Rich Milns

Nginx has proxy capabilities, thought it benefits in the other way than Varnish does. Nginx doesn't cache - so DB and PHP are still loaded in terms of CPU. But Nginx helps Apache to end process faster (fast local connection) and thus save on RAM when multiple visitors (especially when visitors on mobile traffic). And Nginx is transparent, so it doesn't require site re-configured.

Not that Nginx fully replaces Varnish, but it handles high load problem from a different perspective

Unless I'm mistaken, I believe Varnish is more of a caching mechanism whereas Nginx is a higher performance HTTP server than Apache (please correct if I'm wrong here). It was my understanding that Varnish was higher performance than Nginx because it can cache entire web pages so that the DB server and PHP for example do not have to be hit on every request. It also features things like Edge Side Includes and ways of purging the cache using a PURGE HTTP request which is pretty nice from a developer point of view. It's great for high traffic sites, but needs quite a bit of extra configuration for each website it caches I believe. The question is: could Nginx be configured per domain to have some of the features that Varnish seems to offer?

Failover capabilities is not a valid reason. Only varnish does not give you actual failover, you need multiple servesr for that. There are other ways to achieve that using other Parallels' products.

Full page cache,
Also fail over capabilities

I think Varnish is becoming less and less usefull, as there are better alternatives. For Magento there is (for example) Redis, which is a better alternative to speed up your website.

Varnish is also complex to configure and might require different rules per website depending on the content. eg. magento requires its own set of Varnish rules, and WordPress might require yet another ruleset.

I feel Varnish is currently to complex to setup to achieve this in a matter where it is understandable for end-users.

I use it on a VPS but without Plesk Panel. The website flies! It's a huge increase in performance! It would be perfect on Plesk + Ngnix!