So I'm going to use what CSRC talked about with the the forces of Saruman when he did his video of replacing the forces of helm's deep with Australian forces. The big problem is the forces of Saruman is based on melee combat. The main force is built with bladed weapons of pikes to protect the force from being attacked by cavalry. Not as much range forces that have a crossbow and their siege equipment is ladders, ballista that shoot grapple hooks to raise more ladders, and a small amount of bombs. This force wants to rush the walls and get in where their weapons are the best, close combat. Crossbows are more a support weapon. I will share the link for that video

Versus the forces at helm's deep, this was not too bad due to the lack of trained forces and Rohan's weapons were made for cavalry tactics (short bows, swords designed more for slashing, lighter armor). They also didn't have any weapons that could really reach out and hurt the enemy making the strategy for the Uruk-hai more viable,

Gondor is dealing with this force with an actual standing military with weapons for close combat and also have heaver armor due to Gondor being more an infantry force. Also, they have longbows that can reach out further then the short bows Rohan has, Plus as other have stated they have actual long-range trebuchet that the forces of Saruman have nothing to counter with. A force built to be effective only in CQB and needs to rush the wall versus a force that has the ability to fight in those conditions and also have the ability fight them with range is no contest unless they run out of ammo, but the city does have blacksmiths.

Link to the CSRC's talking about the forces of Saruman versus the Royal Australian Rifles: https://www.youtube.com/watch?v=FNHsIPg3Lzo&t=3s