The answer is the first: that structural models provide minimal three-dimensional information. This is because the ball-and-stick model cannot show the actual size of the atoms depicted in the model. You also cannot see the actual bond lengths between the atoms. This, then, limits the accuracy of the model and fails to precisely portray how the chemical compounds being shown are like in all actuality. Thus, though you do see a three-dimensional picture of the chemical compound, this picture is not completely accurate and is not scaled to size.
