Ammonia is great but it's also kind of religated to being it's own thing in the refrigeration industry because of its incompatible with copper/brass which makes it incompatible with almost all standard refrigeration tools and equipment. Basically anyplace that runs an ammonia system will have their own techs because nobody else will work on it. It's pretty common for heavy industrial refrigeration but not much outside of that. It is also deadly in case of large leaks which makes people shy away from it.

CO2 though is already in use for grocery store rack refrigeration systems. Aldi is a big user of them. You also can't actually go below -40C with on a CO2 system without risking forming dry ice in the pipes. As far as efficiency goes, the big impact is from transcritical operation but even then it really doesn't make a big enough impact to negate the benefits. Rack refrigeration systems can already be hilariously efficient with high end equipment and CO2 equipment is always high end. CO2 also has a remarkably high coefficient of expansion compared to most refrigerants which gives you some pretty big efficiency gains in that regard which ballances out most of the losses. As far as leaks go I haven't noticed that they leak any more or less than conventional systems, however when they do leak, CO2 is dirt cheap. The only real downside I've seen about these systems are, transcritical complexity, sheer cost, and for some reason they seem to plug oil filters more often.