That's if you plan them so that all children learn. My daughter was a high achiever, my son was a lazy learner. They both agreed that group projects are great for the teachers because the bright students do the work because the want good grades, the lazy students get the grade without doing anything, and the teachers said they had fewer papers to grade that way.
Good point. I should have stated that they don't reduce the workload IF you do it right.
Some teachers, such as one I had in high school, did not do it right. He did a horrible job.
My gifted son hates group projects. He does tons of work, and other kids do none.