I have not detected anything wrong with mathacademy, it does what it says on the tin. With that said, I stopped using it myself as I felt that my ability to "find the slope intercept form of blankety blank" did not actually translate to any deeper understanding of math than I had before.
I think mathacademy is excellent for developing the skills necessary for solving the problems on mathacademy (which largely are the problems on math exams).
But if your goal is understanding rather than application, then mathacademy is more similar to duolingo than it is to language immersion. You can go quite far in mathacademy (and much of performative mathematics) without really having thought about anything. It is procedure learning rather than understanding.
I think mathacademy is excellent for developing the skills necessary for solving the problems on mathacademy (which largely are the problems on math exams).
But if your goal is understanding rather than application, then mathacademy is more similar to duolingo than it is to language immersion. You can go quite far in mathacademy (and much of performative mathematics) without really having thought about anything. It is procedure learning rather than understanding.