Srila Prabhupada explain why the Buddhist definition of compassion remains incomplete in his commentary on the Caitanya Caritamrta.
It is stated that mercy is one of the qualities of a Buddhist, but mercy is a relative thing. We show our mercy to a subordinate or to one who is suffering more than ourselves. However, if there is a superior person present, the superior person cannot be the object of our mercy. Rather, we are objects for the mercy of the superior person. Therefore showing compassion and mercy is a relative activity. It is not the Absolute Truth. Apart from this, we also must know what actual mercy is. To give a sick man something forbidden for him to eat is not mercy. Rather, it is cruelty. Unless we know what mercy really is, we may create an undesirable situation. If we wish to show real mercy, we will preach Krsna consciousness in order to revive the lost consciousness of human beings, the living entity's original consciousness. Since the Buddhist philosophy does not admit the existence of the spirit soul, the so-called mercy of the Buddhists is defective.