CO2 is not a terrific refrigerant. It requires comparably very high pressures to be effective and has a low critical point reducing their effectiveness in a lot of normal temperature ranges. More modern refrigerants like HFOs usually have a GWP of <1 and don't require anywhere near as high of pressures, with R-454B being around the same as R-410A.
They work really well, with the downside they are very explosive. However stories of airconditioners blowing up are really rare - it seems the risk is probably worth it for the environment.
Nearly all refrigerators/freezers in Europe are isobutane, and have been for 20 years.
https://www.youtube.com/watch?v=uJJvG10tUAw
They're pretty explosive when they have the right mix with air. This can easily happen in, say, a utility closet with a water heater or air handler, or a fridge in a kitchen, etc.
A small leak on the outside unit probably isn't going to be a big risk. A small leak on a unit in a closet with stuff that sometimes makes small sparks...
Thanks! Could've worded it better.
FWIW, one of the biggest negatives to HFOs is that while they rapidly break down in the atmosphere they end up producing trifluoroacetic acid. This stuff then rains back down, and is a polyfluoroalkyl substance (PFAS).