Boys are less willing than girls to talk freely in groups. Girls are less willing than boys to take charge of building products. Students, especially younger students, can get lost in the doing of the project and forget the project's purpose. It is difficult to have long-duration projects with very young students. It is difficult to define goals for a project.
Valid assessment is complex and difficult.
It is difficult to match assessment strategies and learning goals. Unless learning goals are clear, it is impossible to hold students and teachers accountable. Also, in-depth exploration of ideas takes longer than the more familiar broad and superficial survey of concepts. Teachers often feel a need to direct lessons to insure that students get the right information. Teachers frequently give students too much independence without adequately modeling thinking, structuring the situation, or providing feedback.