Looking at some other explanations from this site, it seems like it suffers from the same things people complained about with Khan: a lack of rigour, and oversi

Looking at some other explanations from this site, it seems like it suffers from the same things people complained about with Khan: a lack of rigour, and oversimplification. They have some good visuals and metaphors, but ultimately the 'plain' explanations of mathematical properties are a poor foundation for someone to build upon when they get to higher concepts.