I must admit that the bonus content isn't useable. The car is so big in poly count that it takes forever to load and render. The casino room will always show artifacts (like false shadows) regardless of the render setting. I have not had these problems with any other content.
I have had my share of wierd problems but none of these caused any major mishap, most resolved themselves after saving the scene and restarting P7. Rendering in a seperate thread does have the advantage that a crashing render has little impact on the rest of the program (I have always been able to save the scene).
You do have to remember that the render is much faster and a better quality setting is often selected, this will have a great impact on complex operations (like reflections).