As I see it, it was deadly and effective when it was introduced. Allied aircrew often reported explosions of unknown source then, since they didn't see the fighter. Later, when this type of weapon became known to them, they knew what to watch out for and how to evade, so efficency and deadlyness might have decreased. And of course, flying just below an allied bomber and causing the bomb load to go off by hitting it with incendiary ammo would cause some trouble to the german Nachtjäger...