That is the short answer. Constructivism appears to be a very effective method for learning, but it is merely a method. In order for it to be successful, there still needs to be a thoughtful plan and an effective teacher.
Because the professional football season comes to a close this weekend, I will use a sports analogy. There are two basic defensive schemes used in professional football, the “3-4”, which has three down linemen, four linebackers and four defensive backs, and the “4-3”, which has four down linemen, three linebackers and four defensive backs. Which is better? Unless you have the players who can make the plays, it doesn’t matter which defense you use. The question of which is better is much like asking if Constructivism promotes academic excellence.
Constructivism, without any guidance, is just chaos. Subjects such as language arts, history, social studies or art lend themselves to the kind of latitude that makes the Constructivist approach an excellent option. However, for mathematics, and some aspects of science, the learning objectives require the course of study to be more focused. It requires the teacher build lesson plans and has examples that go beyond solving the equation. For example, the teacher must have lessons that have some real world problem that needs to be solved using the skill that is being taught.
This is not meant to be a criticism or indictment of Constructivism. I think it is a very good method for some subjects. However, it is not a panacea. It is a method that will not be successful without a competent teacher.