Why is Flags Enumeration considered an intermediate skill?

Question

I was reading this article: Designing Flags Enumerations @ msdn and it says

Combining flags enumeration values is an intermediate skill that should not be required for developers implementing common scenarios.

I think flags enumeration are awesome - something I learned about during my experience with C.

Why does it seem like this article is cautioning developers away from what I consider to be a useful (and not complex) skill?