A punctured envelope can result in high oxygen hydrogen mix at the point of failure, and static or sparks can ignite the mix, leading to a failure cascade. Hydrogen oxygen mixed burn really hot, and you want envelope materials to be super thin and light. Cellular designs that are engineered around controlling catastrophic failure of individual nodes might be the only feasible way hydrogen ever becomes practical for anything near people. That increases the weight since you need lots of little envelopes instead of one big balloon.
Fortunately, materials science is allowing for this, and safety features in big helium ships are always improving and laying a foundation for the eventual transition to hydrogen.
Fortunately, materials science is allowing for this, and safety features in big helium ships are always improving and laying a foundation for the eventual transition to hydrogen.