All hosting services include the Varnish service. Our support will ensure the Varnish service is started, properly placed into your request flow (including support for SSL requests), and your Magento VCL configuration file is properly linked.
Once Varnish is in the request flow many things can and do go wrong. Content that should not be cached, is cached. Content that should be cached, is not cached. This is not our responsibility. What is cached, and what is not cached, is in the individual configuration files for your theme and the extensions you are using. It is your responsibility to adjust these configurations to ensure Varnish is caching your content properly.