Pros
Have you ever spent the majority of your time reviewing how to multiply or divide fractions when you were supposed to teach conic sections? If you've ever taught high school math, you perhaps know what I am talking about. In comparison, AoPS is among the very few pre-college schools where authentic math is taught. Why are parabolas symmetric? Why are there infinitely many prime numbers? Why do we have 1/3 in the volume formula of a pyramid? These are among the questions you can dive into when teaching at AoPS. Many AoPS employees left their jobs at academia, schools, financial and tech companies to pursue their dream in math education. It is a company where capable people come together for a shared goal. Note that AoPS is expanding to the area of Chemistry, Language Arts, and Physics. So it is now more than just math.
Cons
Nothing really. Generally speaking, you need to share the same goals of the company in order to enjoy working there. This is particularly true when you work for a small company with a unique identity.