The main criticism of flat organizational structure is that management will develop as an emergent property anyway, but it will do so poorly. "The Tyranny of Structurelessness is the classic essay on the subject: https://www.jofreeman.com/joreen/tyranny.htm

A poor manager in a structureless org is only going to have as much influence as their popularity allows. While this may cause damage, it pales in comparison to a bad manager in a traditional hierarchy who has the power to force their decisions.