"Since the British troops were able to protect themselves behind the ridge selected by Wellington, French guns did little damage. On the whole artillery was not an important factor in the battle of Waterloo."
Adkin estimates that 2/3 of Anglo-Allied casualties (11000) were caused by French artillery, and that is despite the use of the reverse slope.
He estimates that 45% of French casualties were from Allied artillery – 140000 men. This is skewed by the fighting around LHS and Hougoumont where the defenders were able to cause greater numbers of casualties by musketry.
If the Anglo-allied artillery had no enemy artillery to worry about, it could be tearing holes in the phalanx. It could be concentrated and little need to keep batteries in reserve.
Napoleonic columns were deep but there were formed of a succession of 3 deep lines. There were gaps between the lines – balls might hit several lines but might bounce over others. There was also the possibility of dodging to one side. Additionally, shrapnel would likely only affect one line.
Alexandrian phalanxes were solid blocks. A shrapnel burst in the middle would have far more men in the danger zone than a Napoleonic column.
Adkin estimates that one casualty was caused for every 1.5 rounds fired (which apparently agrees with studies from the era and later). If firing at Alexander's denser formations, these numbers would likely have been even more significant.
The phalanx is also reckoned to have relied on its cohesion for its strength. With great chunks being taken out by each burst of artillery fire, cohesion would have suffered. Across much of the Anglo-Allied front there were also terrain features to disorder a phalanx.
Once within musketry range, the phalangites in the rear are further disordered by having to step over the dead of the front ranks. This would have been more of an issue for them than a Napoleonic army again because of the need for cohesion and the density of the formation. It was considered an obstacle even at the time of the Battle of Magnesia.