It doesn't cause metals to become brittle. Its relatively stable. It doesn't require as low temperature or as high pressure to liquidize. It also stores ~50% more hydrogen per volume, as each ammonia has 3 hydrogen, unlike elemental hydrogen with just 2.
But we have handled ammonia on an industrial scale for more than a century and are really good at it. Any safety concerns about ammonia are ridiculous when compared with some of the other substances we handle and live around on a daily basis.
A a John Deere employee I can tell you that several very much wanted products have failed to be developed because someone put their safety black-hat on and came up with a to abuse that product the release ammonia into the air. Once an evil person figures out how to control our system it is trivial for them to figure out many terrorist attacks involving ammonia releases.
Yes the world deals with ammonia all the time. However we have special training for anyone who handles it. Even the most caution to the wind types wear full respirators and thick gloves when handling it.
When you buy ammonia at walmart what you get is 1% ammonia, 99% water. Then you are instructed to dilute it with more water 16:1, Even at that ratio it is nasty enough that those who use it have windows open.
Please be specific. Liquid anhydrous ammonia is a common fertilizer. John Deere still sells e.g. the 2430 and the 2510. How did the hypothetical "very much wanted" products differ from those applicators? Are we talking about a handheld model? (That might have been a bad idea!) One suspects the hypothetical "evil persons" whose threat delayed product development were more interested in cooking meth than in terrorist attacks...
John Deere is to be commended, one supposes, for being much more concerned about blackhat terrorists than e.g. Siemens with all their internet-accessible SCADA installations, but neither ISIS nor Putin care about killing a couple of farmers in western Iowa. Thread parent was about automobiles, and while maybe professionally-operated vehicles like semi trucks or buses would be better with which to start using ammonia on a large scale, vehicles are unlike applicator implements in that they aren't actually designed to release ammonia into the environment. Conceivably a car could be hacked to lock the brakes at high speed, but there's just no mechanism to pour out its fuel tank.
Ammonia is less flammable and explosive than gasoline, which is commonly found in currently-engineered and currently-existing fuel tanks. They are both much less flammable and explosive than hydrogen, which is the other fuel under discussion in this thread. Ammonia tanks would be pressurized, which is different than gasoline, but lots of tanks are pressurized.
Hydrogen is not toxic by inhalation, but it's a much worse explosion/fire risk.
Hydrogen is flammable when mixed with air between 4% and 75%, and it takes a minimum energy of 0.016 millijoules to ignite. Ammonia is flammable between 15% and 28% and takes 680 millijoules to ignite. It takes much more energy to ignite ammonia and there's a much narrower range of mixtures with air where it can support combustion.
It doesn't cause metals to become brittle. Its relatively stable. It doesn't require as low temperature or as high pressure to liquidize. It also stores ~50% more hydrogen per volume, as each ammonia has 3 hydrogen, unlike elemental hydrogen with just 2.