One of the most compelling features of the original MvcIntegrationTestFramework was that the assertions were performed server-side (i.e. in the server AppDomain). That meant the test code could reach into the ResultExecutedContext and, for example, pull out the ViewData.Model with strong typing and verify its contents.
It took me 10 minutes of browsing through the code before I realized this. It would be nice to explain up-front that this was the trade for increased simplicity.