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.
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.