The downside is, you had better have a good team developing this because you will need to have almost everything an architect needs to use ready all by yourself. This allows you to integrate a lot of functions because you can be sure that all entities created/imported by your application has been properly 'prepared' and their associated parameters properly indexed and stored. Working as standalone application has its advantages because you are in total control of how users use the model, and you do not have to make room to capture things that are not created by your application.
I've heard that Revit is really good by the way.
I haven't tried Chief Architect and Revit however looking from the videos, they are standalone applications.
