The key to a Soul Attack is its suddenness. They are most effective when unexpected. To achieve an instant kill, the power gap between the combatants must be fairly large. If the target possesses Martial Arts that defend against Soul Attacks, or if their own Soul Power is also strong, the attack's effectiveness will be greatly diminished.
Generally speaking, Cultivators don't specifically practice Martial Arts to defend against the soul, as Soul Attacks are relatively rare. One only has to look at the Soaring Dragon Empire for proof. Only the Divine Seal Sect has mastered Martial Arts involving Soul Attacks, and they are incredibly difficult to cultivate.
