They cannot be used against walls, but make excellent and inexpensive anti-personnel machines. The catapulta is a direct-fire weapon, and is especially effective in great numbers. The cheirobalista, developed for use by the Romans, is the last great development in small, long range torsion arrow-throwers. The onager is not as accurate as other torsion machines, but made up for this shortcoming by being very inexpensive and simple to construct. Less can be done to disassemble it for easy handling.
Onagers make inexpensive direct fire machines.
The trebuchet appears late on the scene in Western Europe but has ancient roots in China. The trebuchet is not portable in the medium to large size ranges, but large size is the trebuchet's strength since if can be built far larger than most siege engines, even a 3-talent ballista. The trebuchet is very simple to construct, and can throw much heavier projectiles, so it is ideally suited to smashing walls. It is not used primarily as an antipersonnel machine. It is an indirect, or high ballistic trajectory, machine.