AoPS leans a little too hard into silly contest math / trivia tricks, not into deep understanding of the underlying mathematics. Also, they have low production values, lacking the nice graphics used in professional school textbooks (before Eureka and the greedy cult of throwaway monochrome textbooks ruined everything) to help students visualize 2D/3D functions and geometry.

The AoPS books jassume that if you are smart enough to learn the tricks and hard problems, you are smart enough to learn deep math later.

Which is a strange premise, because the PhD geniuses who created AoPS, base their credibility on the fact that they aced all these contests when they were kids... But they did it without any of these materials!

They created a "teaching to the test" curriculum, which, while mathematically sound, created a rat race that makes the contests harder every year, requiring more memorization and silly speed training, as more people memorize all the strategize that the original contests expected student to try to creatively discover on their own.