Are passive piezo speaker more reliable than magnetic ones?
since your question is in general terms, with no specific numbers such as required lifetime, humidity levels, shock levels, etc. , one can only answer in general terms. Magnetic (voice coil) speakers are reliable (again, no numbers) but the vibrating membrane poses a potential fault problem since it is fragile compared to a piezo speaker which has none. However piezo materials are relatively brittle and can shatter or crack when exposed to shock. Also note that a magnetic speaker probably won't fail completely even if the membrane tears but a piezo speaker will stop working if it shatters.