Ullage motors have to be solid - this way they don't need ullage themselves to fire - or to be fed from separate tanks - this is another reasonably complex subs

Ullage motors have to be solid - this way they don't need ullage themselves to fire - or to be fed from separate tanks - this is another reasonably complex subsystem which needs to be refueled somehow between flights. If ullage motors are solid, that needs to be re-loaded between flights. So, overall the ullage system is a certain complexity to design, build, refuel, a weight to carry in flight - clearly some drawbacks. Hot firing simplifies things in this regard.