Flavius, it's accuracy more than anything. Most of the artillery is rather inaccurate, but the degree of spread is related to the range to target. The farther off it is, the farther off target a round can land. Since trebuchets outrange catapults by a large margin, they are far less accurate at the edge of their range than catapults are at theirs. You have to bring cats into the range of missile troops with the "long range missiles" tag to be able to shoot at them, while you don't with trebuchets. However, there's been a couple of battles where I've had missile duels between 4 units of pavise crossbow militia on my side and 4 units of trebuchets on the part of the enemy, and I won. The trebs are so inaccurate that they can't seem to hit a thin line of troops except rarely. When they DO hit, it causes a lot of casualties, but I typically will only get hit once or twice total during the exchange. Ballistae are a LOT more accurate.

Me, I like trebuchets against walls, but I don't start dragging any artillery to a field battle until post-gunpowder, unless I just happen to get caught by an army marching towards a settlement I intend to take with it.

Now, after gunpowder, I'll bring them along sometimes. Not always, not even regularly, just sometimes. I use a couple of units of culverins/cannons/basilisks to counter the artillery spam the AI likes so much. I've never yet played a faction that had serpentines. The range on culverins and basilisks is just insane. They also make quick work of walls and towers.