AFAIU the main challenges is that the thermophysical properties of CO2 are different than for the usual fluorocarbon refrigerants, so it needs a different desig

AFAIU the main challenges is that the thermophysical properties of CO2 are different than for the usual fluorocarbon refrigerants, so it needs a different design taking that into account (among others, needs to handle much higher pressures). The advantage of the fluorocarbon refrigerants is that the current automotive one, R1234yf, is mostly a drop-in replacement of the previous R134a, which in turn was mostly a drop-in replacement for the freon refrigerants (R12) that were banned due to destruction of the ozone layer. So going down this path manufacturers can basically slightly tweak their original designs, potentially originally dating back to the beginnings of the automotive AC era. So saves on R&D costs. The downside is of course that these fluorocarbon refrigerants are very expensive compared to CO2, and the ever tightening F-gas regulations around the world meaning there seems to be a never-ending rat race where a fluorocarbon refrigerant gets banned, necessitating the development of a replacement which is more expensive, and then in turn gets banned in a decade or so. Rinse and repeat.