Mathacademy is meeting a market demand, which mostly seems to be being good enough to solve problems in several standardized math courses. In that regard, I think it is a thing that I would prescribe to any student (along with tutoring and study groups) in order to do well in that course.

But I am just not convinced it can ever (especially, in isolation, which you never claimed) lead to you being a good mathematician in the sense of having internalized the material. I can't quite put my finger on it, but I think it is similar to the feeling you get when you are some high level in duolingo and realize you still can't follow daytime soaps. You have the rigid intelligence, but not the fluid (internalized?) one.