Im my opinion Mendix should be used to create dynamic web applications and not static websites. So if you are trying to build some kind of application e.g. an online mail client, online reporting tool, online some other tool mendix is the way to go. But if you are hosting a website such as www.mendix.com then you should not use mendix, or atleast not for the static parts of the website.
In addition to that if you use mendix to create an open website you are required to enable annoymous users and because it is a open webpage they will eat your concurent users rather quick. Also Mendix has a lot of widgets available in the app store but most of them are focused on business applications rather than use on an open website. All in all i don't think that you would want to build an open (static) website using Mendix.
I would say it's a matter of using the right tools for the right job.
The power of Mendix is in the visual design, integrity, speed, business logic, security, integration and moving along with your business instead of lagging behind.
Don't compare it with dedicated systems like CMS-es, webshops, document management systems built for speed, performance and flexibility.
Edit: Mendix can handle thousands of concurrent users, as long as the hardware support its. Ask Mendix for official reports.