What an interesting topic.

In my opinion, what produces the success of this type of work schemes are two things: first, the number of team members is always small; rarely does it exceed 6 people, maximum 8, more from there the team is divided. The second thing that makes this type of teams efficient is that all the members are related peers.

I agree on that with you and maybe even smaller groups I'd say, but obviously that depends on the project. It's difficult to work in a large group and results are not the same. I'm talking from experience.

Responsibilities must be individual, and so everyone will be held accountable for their actions.

This is another excellent point. Some think because it's a group work, they can relax and let others do the work. That is very wrong.