If I recall, Fusible plugs really are not for pressure relief due to a fan failure or plugged condenser. A mechanical relief valve is meant for that. A fusible plug is really designed for a unit located in a fire. This would allow the refrigerant to blow off before an excessive amount of pressure could rupture the receiver/system and cause injury or death. Its true that they sometimes fail due to high pressures but originally it was a UL thing for fire, If I recall my facts correctly. I can look at my old Sam's if this discussion goes further.
Rich