"If engines leak unburned ammonia, that can be toxic."Applies to...

"If engines leak unburned ammonia, that can be toxic."
Applies to any fuel. Leaks are very easily monitored nowadays. Even down the valve in your fuel cap.

"And ammonia engines tend to produce nitrogen oxide"
Already been dealt with in everyday motoring. That's what adblue is for.

"also a potent greenhouse gas."
Incorrect. 'ammonia is environmentally compatible. It does not deplete the ozone layer and does not contribute to global warming.'
A GWP of zero.

"Ammonia burns slower and is harder to ignite than fossil fuels; most ammonia engines need a dose of diesel or hydrogen to get them going."
Yes, ammonia is perhaps not the best fuel. But it's early days. LPG/diesel have the same ignition problems, always a small amount of diesel added as LPG is unlikely to ignite under compression alone, while diesel will.