Here's what consumers have asked with answers from Khan Academy staff and previous consumers.
It is not a solution designed for exams. Exam preparation is a whole different world. You need to consider that at Khan academy the information is not arranged in a way to fulfill all the criteria in SAT. It is helpful to illustrate some baffling points, especially in math, but one must not limit one's self to the content of Khan Academy.
There is an old adage in education , that 'Spaced' learning is more effective than 'Clumped' learning. Waiting for the next Mastery challenge teaches patience, allows you to practice more skills in the dashboard, so the mastery challenges have more to do assessment on, and as you note, ensure that the concepts are transferring from short-term, to long term memory in your brain.