A sibling comment mentions scrubbing NOx emissions as a significant issue, and it strikes me that the shipping/cruise industry is already a major polluter in that area. (Unlike cars, which must have catalytic converters because of local landlubber laws.)

Why would you want to burn ammonia in an electric vehicle?

I'm not sure I'd leap to "much simpler storage problems than for hydrogen" for a highly corrosive gas.

The lower explosive limit of hydrogen is ~4%. By comparison the 300 ppm immediate danger to life and health threshold of ammonia is .03%.

It is intrinsically dangerous, i.e. without a source of ignition, at concentrations 2 orders of magnitude lower than the LEL of hydrogen.

Not every hydrogen leak is a concern, but just about every ammonia leak is.

The established OSHA 15 minute exposure limit for ammonia is 35 ppm, 8 hours is 25 ppm.

Yes, fuel cells need more usage as we move away from oil & unnatural gas.

Filter the exhaust through a caustic solution to neutralize NOxs.