I think it's more about the damage output. Five shamans is viable in 5v5 because five attacks can burn through 10-15K HP pretty quickly, at which point you escape and do it again a short time later.

In 2v2 or 3v3, everyone has the same amount of HP, but you have less shamans and can't burn someone down anymore. That makes the fights longer and more dependent on kiting and CC, which a multibox group is terrible at.