Well, hydrogen tanks are kept at crazy crazy pressures, so if you stand next to an active leak it will literally cut you in half - high pressure tanks are pretty scary stuff. On the other hand, pure ammonia will kill you straight away if you breathe it in. Diesel is not great, but short of drinking it it won't kill you, and it doesn't really like exploding either. I'm not really trying to make an argument against hydrogen/ammonia, I just find this stuff interesting.