Ammonia, the most likely carrier you're talking about has its own set of issues. It burns rather cool so will require new engine designs to overcome this. Also the engine must run efficiently or will produce nitrous oxide gases, hence smog. Lastly, and one of the bigger risk for end users is ammonia is a highly aggressive irritant. While gasoline/diesel can be bothersome in low concentrations, ammonia will send you running and choking. I personally think it will lead to consumers being more afraid of it and lead to adoption issues.

You mean a liquid hydrogen carrier pathway like liquid ammonia. That seems possible, but requires extra plants and equipment, and obviously you still have to store H2 if you want to burn it in an engine in a vehicle.