As a non-USA teacher, I find it frustrating to have preset missions that I can't change. I, like most teachers, have a great deal of rigidity in the curriculum I teach.
I would love to be able to use Mastery Challenges in a mission full of exercises that I set. In other words, exercises that are relevant to my students. It is difficult getting my less motivated students to 'buy in' to this amazing website when they are continually asked questions on topics we haven't studied (and won't) or questions on ridiculously easy topics like counting or telling the time.