All I can say is that it worked really well in my DCs' school.
They understood math at a level I did not at the time I was in school. They were taught every topic in depth. There were times I felt they were going really, really slowly through the material but there was method to the madness. Their elementary school produced students who were comfortable with their peers in their classrooms and in their interactions with their teachers.
The idea that an elite should be created in math, and pushed forward, while the rest are allowed to lag behind and then drop out is not one that is accepted in American schools. Math is considered to be something anyone can understand and demonstrate competence at if they work hard at it. It's a more democratic approach, and because math is one of the core intellectual endeavours American students are expected to gain competence in and not just drop, teachers take the competence of every student seriously.
This is also why high schools offer so many math tracks. Not every student has the interest or the need to do calculus but there is plenty of material aside from that to use to develop the ability to learn and practice the virtues American education values, like self advocacy, consistent effort, and prompt attention to problems.
The GPA system means students need to work consistently every day in high school, and not just rely on exams as evidence of competence. The GPA is a measure of attainment that illustrates how well a student has learned to learn, and how much resilience amd perseverance a student has, as well as a student's level of subject mastery. It takes a good deal of maturity to hand in accurate homework every day, study for weekly tests, and contribute in class every day for the four years of high school. Students have to ask for help if there's something they don't understand, and make sure they monitor their own learning in order to keep daily grades up.
The goal of learning to learn, and associated skills like communication, self advocacy, and building of personal confidence are emphasised in American elementary schools, not the narrower goal of mastering a certain body of knowledge. A child in primary school could be very quick to understand a topic in math but might not have the resilience or the confidence to attack more complex math at secondary level, or ask for help if floundering, or might feel a failure if help was needed in a system that values mastery above all and separates kids into groups according to perceived ability at a very young age.