Surely the hardness of the metal must matter?
This. I have a jacket with one snap that failed like this. Male is stuck in female (weird, lol). Looking for quality solid brass replacement.What holds the snap closed is the spring in the female part, which will always be mate from steel.
I don't think the material of the snap really makes that much difference.
Like @Edward said, failure is usually caused by a bad riveting job, causing one of the part to separate from the leather, not the male nipple getting worn to the point where it doesn't clip securely into the female part.
(that sounds weird...)