Say 32 oz of refrigerant per car, 675x CO2 equivalence, if your car AC vents all the refrigerant it's like releasing 0.675 tons of CO2.

EPA says an average passenger vehicle releases 5 tons of CO2 per year, so if your AC lasts 10 years it's about 1% of your car's greenhouse gas emissions.

Leaks in a larger building AC would be worse, but I bet all the vibration and getting banged around by potholes makes the car systems a lot more prone to leaks than a stationary unit.

AFAIK Vehicles use flexible rubber pipes because the rigid copper pipes of fixed split ACs would not withstand the vibrations of a car. The downside of these more flexible pipes is that they leak more.