Indeed, the way catapults works in Warhammer is VERY fictious. In Warhammer, stone throwers works like the equivalent of modern day artillery.
The mortar could work like that, since it was designed to have a shell that explodes and sent shrapnel flying. But catapults mostly threw rocks. Indeed, experiments have shown that you can fire a bucket of smaller stones, instead of the big one, and there is even some archaeological observations from Eketorp fort at Öland indicating a possible use of a light stone thrower as an indirect area weapon to help cover a gateway in an outer wall.
But it is very limited in that sense. It was most likely prepared and aimed at the gate as preparation for an eventual attack, not in the heat of the moment. It was a fixed weapon, not a field weapon.
You could throw fire and such, and the romans did on occasions, but it was rare to have heavy catapults in the field. Bolt throwers, yes, rock throwing catapults and trebuchets, no.
They where siege weapons, not battlefield weapons. And in sieges, their main task was to throw large blocks of rock, aiming to destroy structures rather than targetting personnell, weakening walls for example.
And wether a rock would shatter or not on impact depends heavily upon what it impacts upon.
The only rock type I think would shatter and with pretty lethal result, is flint/firestone, and that would send razorsharp splinters in every direction, but I have no clear evidence for it´s use as a specific ammunition in a historical context.
So, you cannot really look at history to judge the use of catapults and mortars in warhammer, since they are used in a very made-up way akin to modern day artillery.
Pretty odd is it. You can use catapults in a rather bizzare way in warhammer, but hell no, you cannot have your archers fire a proper arrowhail in several ranks.
Well, everything to make strange gizmos needed, I guess. The warhammer races are all to stupid to use archery in an effective way.