It has to be mixed with diesel in order to function in compression ignition engines, otherwise known as "diesel engines". So, this "criticism" could be leveled at gasoline and other fuels as well. Spark plugs will ignite ammonia just fine. There is a single reference to this in a different post on the site. [0] My understanding is that most "hobbyist" ammonia fuel setups (i.e. "my pickup runs on ammonia now!") also use spark ignition. I suspect they're investigating the diesel engine possibilities because they would require fewer modifications to existing large marine engines.

[0] https://www.ammoniaenergy.org/articles/wartsila-tests-intern...