I don't think it fair to make any such claims of "lack of effect" or the like.
Considering that the defenders of Hogwarts were few, and the aerial views of the battlefield, before the clash, showed what must have been a few thousand "villains." Flashing forward to the following dawn, where the tattered and bedraggled survivors of Voldemort's warriors had approached the main gate, showed that their losses had been "legion," to coin a pun.
We had been able to get accurate impressions of Voldemort's supporters, before the opening of hostilities, from those aerial views. Remember, we are able to count both the minions physically with Voldemort, who had been contributing to the destruction of the defensive barrier; as well as the horde of virtual "wild were-animals" who had approached the bridge over the gorge.
Voldemort had been able to field not only turncoat magic-wielders and criminals lately freed from prison, plus beings such as werewolves, as well as non-human magical beings like the vicious giants, and even dark creatures that are essentially "cannon-fodder," like the giant spiders from the heart of the Forest.
The aerial views of the environs of Hogwarts as the "Dark Forces" advanced upon the defenders made it very evident that there had been thousands of Dark Warriors.
Then, battle was joined. It was obvious that the "clever lads" had properly booby-trapped the covered bridge, leading to the demise of hundreds of those that some have been calling the "Savage Horde." The horde had evidently been largely composed of beasts like werewolves as well as the "juvenile-delinquent witches" or "punk thugs with wands," who seemed to have sprung up during Voldemort's reign.
Very few werewolves were seen, during the later battle for Hogwarts; and in fact, the morning's survivors who had assembled in the far end of the courtyard was clearly a rather small fraction of those who had come swarming through the woods, or who had stood with Voldemort, before the onset of battle.
DID "bombing" the attackers with Mandrake plants "help" by slaying large numbers of Voldemort followers?
I great deal has been said about the noise or din of battle. I feel, however, that there are many factors which have not been considered in the debate.
I should like to point out that creatures such as werewolves, among others, have supernaturally-acute hearing.
Plus, there's really no information as to how well--or how far--the shrieking of fully-developed Mandrakes is lethally-effective. It is not a subject that frequently comes to the fore.
If the defenders reached the wall unremarked by those below, they could have lobbed Mandrakes into clusters of villains, initially at their leisure.
It must be remembered that this was nothing like a "regular" battlefield: With the exception of "explosive spells" that had caused something akin to an explosion.
There was nothing like the echoing rattle of gunfire, the loud explosive "Bang" of rocket-propelled grenades, or the hollow echoing detonation of thrown explosive ordnance to add to the overall battle-noise.
There would only be the shouts of spell-words or phrases, and the "bang" of intermittent or occasional "explosive" spells, and other human- or creature-made vocal-outbursts. It would actually be a "quieter" battlefield that what modern soldiers would routinely face.
In a scene such as this, the "kill-radius" of Mandrake screams could be rather large. Eight or ten such "bombs" delivered from the top of the wall could easily slay upwards of a hundred or more attackers.
This contention could be borne out by comparing the crowds of supporters who had gathered about Voldemort, before the attack on the magical "shield" had begun, combined with the comparatively huge numbers composing the "Horde" that attacked Hogwarts from the playing-field side.
Both of those scenes clearly indicated that Voldemort had hundreds, if not thousands, of fighters.
But when Voldemort and his followers approached the main gate of Hogwarts the following dawn, well over half--possibly as many as three-quarters or more--of his combatants were not in evidence.
During the intervening night, somewhat between fifty and seventy-five percent of his forces had evidently been slain.
Most tellingly, these losses included all of his "heavy forces"--- there had been "naught but wand-wielders left," so to speak.
It was also abundantly obvious that a great many of the remaining Voldemort supporters were "rather loose about the belt." As a soldier would say, they were greatly lacking of guts; or courage.
As soon as Potter "resurrected" and bounded off, and Hogwarts defenders took up the joyous cry of "Harry's Alive!" dusky plumes of smoky magical energy leaped up as several supposedly-staunch Voldemort supporters deserted with alacrity.
And all this, while Voldemort was not only alive and alert, but close to hand!
And of course, supposedly trustworthy Lucius Malfoy, wife and son in hand, had turned tail and were legging it, homeward-bound across the causeway, before Harry had even crossed the courtyard and rejoined his friends.
As at any real battle, action had become general across the entire front, and the scattered--and sparse--Voldemort supporters were pitching it to and fro with the Hogwart contingent until Longbottom put paid to the snake, and Harry had given at-last-mortal Voldemort his comeuppance.
Once Voldemort and his final horcrux had been dispensed with, the few remaining foes had no heart to go on.
But it is evident that Voldemort's initial numerical superiority had been very seriously eroded; not just during the night, but prior to Voldemort's offer of a respite to allow the defenders to gather their dead.
Battle had not resumed until after Harry "resurrected," and still the visual appearance of the Voldemort contingent was hardly a quarter of what it had been, before fighting commenced.
SOMETHING had slain them; if this had included upwards of a hundred laid low, the victims of Mandrakes' screams, then there it is.