True.
The issue I see is how to get the goods to return to the bridge(s) safely, reliably, and quickly. Some creatures are very large, or have a high agility, and can theoretically make it through. This means the hell-yard needs to be a relentless autorepeater, with accumulating gibbed body parts and goblinite pieces in constant motion.
A raising bridge on the ground will hurl objects up into the air, where they fall down again. The problem then is having them fall under triggered bridges before they reset, and the resulting atom smash. Channeling under the bridges requires supports under one of the bridge's edges, or it will discombobulate spectacularly, rather than fire continuously.
The idea here, is to have a singe "control room" with 2 levers. One lever dumps a metered volume of water down 1 z level into another, larger room with pressure plates and a hatch cover on the floor. This activates the autofire mode, since the pressure plates are what drive the catapults, and the measured volume of the room above makes a 2/3 depth with scattered 3/3 depth water whizzing about. When 3/3 is over a plate, it triggers. This triggers the plates far faster than a creature repeater. The second lever in the control room drains the autocontrol system, shutting off bridge activations. This makes it easily on/offable, and continuous fire.
(Eg, we have a 4x4 tile room above with 4 hatches tied to the "on" lever that fills with 7/7 water from a floodgate that shuts when the hatches open. This dumps 121 units of water into a 5x5 room. The water depth is 4/7, with some 5/7 tiles whirling around. We can have 24 pressure plates in the floor of this chamber set to go off on 5 depth liquid, and 1 drain hatch. The speed that the 5/7 water tiles whirl around is faster than nearly any creature's movement speed.)
A tall wall around the hell-yard will prevent goblinite bombs from escaping. A room perched above the hellyard with drop holes directly above the bouncing betty bridges, cycled with a retracting bridge doubles the pleasure, and ensures the bouncing betties stay armed. This room can be several Z up, to avoid goblin crossbow fire.
This would exploit the stupidity of the goblin ai, where the subordinates all huddle around the leader. 1x1 betty launchers would toss individual units, (and debris) causing them to land on their peers, resulting in stunning, and major injuries. Stunned creatures can't dodge, so the murderhole dropped items can't be dodged.