My understanding is – smart cruise is the one that can be less efficient – if on a motorway and you have set it to 70 and the traffic is reasonable heavy so the speed of the car in front is constantly changing and dropping below 70 – hence your car is braking and accelerating to match.
I find smart cruise helpful – especially on a busy motorway and not bothered about being slightly less efficient.
It certainly helps my knees to be less painful and I can wriggle and stretch out more.