mythbusters proved years ago that the golden fancy shiny paint they painted on the hindenberg and static electricity was what caused the main disaster of it. several tons of thermite will do that to you. the hydrogen itself, has a very fussy air/fuel mix ratio that makes runaway chain reactions tricky and is why hydrogen gas powered engines are not a thing. very hot, high compression ratio, and the tricky part is flashback arresting because the tubes are connected to the tank. but a hydrogen balloon is really not that dangerous if the material containing it isn't fucking thermite. but it's all a bit too late for that.