Students complain the structure is not as organized as they expect. None I don't think I have noticed any decline in the skill or the learning of mathematics as a result of group learning. My only problem is dealing with occasional dysfunctional groups effectively. Very good students are frustrated and feel held back by the slower students in their group. I worry we might lose some budding mathematicians this way.
Freeloaders can hang on a little longer before they drop.
Students who resist participation and who resist really trying to understand, vs. memorizing something. Really good students don't get the topic coverage that they would be exposed to under a more traditional approach. This is a detriment to them. Students sometimes don't practice manipulative skills as much as they really need to succeed at high levels. But, most will never go on to graduate mathematics work anyway from here.