We just need to choose the best electrode for the fuel cells. So far optimizing for NH3 has not been as well researched as for H2 and other fuels.
NH3 "burned" in a fuel cell is converted back to N2 and released to form air again. Although the N2 weighs more than the H2 it helps to transport, it greatly simplifies storage -- liquid NH3 storage is not that more challenging than propane storage -- and should ultimately reduce transport costs. However, the best applications for NH3 as fuel may very well be large seagoing ships, as well as locomotives. Building storage and distribution facilities for personal vehicles would be considerably more challenging economically, but could still pay off in the long term.
The US gov't is already invested in this idea -- see this link for example.
Just hope we can eventually find electrode materials that have less environmental impact than, say, lithium-ion batteries.