They're everybody's weakness, man.
Especially Bullet's. Bullet loves penetrating ghouls of all genders and orientations. However, bullet prefers to penetrate the cranium to get to the juicy squishy brains within.
Bullet's cousin, Plasma Blast, also has a fondness for ghoul cranium, but rather than penetration, PB prefers to envelop the ghoul in molten hot love.