It would be best if the system were designed for HCs, but they do seem to work well in practice. From what I read, the system won't get as cold at idle as with R12, but people seem to be happy with the results.

I'm more worried about R744 (CO2) as a refrigerant to be adopted by car manufacturers and its inherent dangers due to extremely high pressures. HCs have shown enough promise that I would rather the manufacturers go that route. A properly-designed HC system would chill you out of the car at all speeds.