Here's the thing. Zeppelins were really, really, really, really, really hard to shoot down. They were super light, and bare of all but necessities like only around 2-4 anti-aircraft guns and only enough food/water to last a day or 2 if that. That being said, lets talk about how to shoot them down. They were delicate, yes, but that was only if you could get high enough to actually hit them. During the early and mid phase of the war, Zeppelins were leagues ahead of airplanes technologically speaking. Airplanes couldnt get anywhere close to the top height a Zeppelin could climb. Along with that, artillery shells couldnt get that high either, and usually exploded hundreds of feet below the airship. If a Zeppelin actually flew low enough to be hit, normal machine gun bullets just wouldnt cut it. The hydrogen was both in multiple gas bags AND wasn't pressurized, so if a Zeppelin was hit, the hydrogen would take hours to leak out of just 1 gas bag. That was out of 10-16 gas bags. Incindiery bullets were developed, but without oxygen in the gas bags, they passed through harmlessly without a single spark. 3 different types of bullets were produced to fix this problem. A special explosive bullet would detonate if it hit the frame inside, and would blow a small hole in the canvas skin. At that point, airplane gunners would focus fire on that point, alternating between different incendiary bullets until a spark would start a fire. IF the Zeppelin caught fire, that was the end of it. It was just a matter of actually starting the fire. Zeppelin raids continued throughout the war, but slowed down significantly after the incendiary ammo was developed in 1916. By 1917, the raids were continued by the Zeppelin-Staaken company. The result: the Gotha Bomber, the largest airplane in the Great War. The Gotha Bomber could carry more bombs than the Zeppelin, cost less to make, and were much harder to shoot down. They carried roughly the same amount of crew members, with around 7 walking around on the wings in mid-flight to fix the engines, repair bullet holes, and manually drop the bombs if need be. Along with the engineers, there were usually around 2-4 gunners and a pilot. tldr: they were extremely cost effective from 1914-1916 when there were literally no air defenses, but when explosive ammunition was invented and developed, from 1916-1918 the Zeppelin was mostly replaced by heavy Gotha Bomber airplanes.