"There are many things Mathematica can do that Sage cannot (I provide examples in this article)."

I'm not surprised, but I would like to see a link there to some of the instances. So I went looking. After a minute or two I found an example in the article, a differential equation. http://arachnoid.com/sage/terminal_velocity.html#Falling_in_...

In my understanding of the mission of the Sage project, it is valid and helpful to report each such instance as a bug in Sage.