The best way to ensure good content is to ensure smart teachers, which is what these metrics are designed for.

Fractions and decimals are not difficult. I doubt that kids who have been learning math with Khan have problems understanding them. But ìf Khan has been a little unclear for some percentage of students, the addition of a few exercises should solve the problem. The lacunae of a smart teacher are easily fixed, unlike the confusion caused by a teacher who doesn't understand the material.