I’m with edukator on this.
Driving attentively I can get better economy and more consistent speed without cruise control because I can see the inclines and the curves and the slower traffic and the gap four cars back in the middle lane that I’m going to move over into when that range rover thunders past in 20 seconds time. I can feather the throttle to make that 2mph speed difference that means not getting too close to the lorry in front without braking etc.
I’ve yet to drive a car that has cruise control that can cope with all that.
Also when very little else is being driven that consistently it can work against achieving good flow in the traffic.
When it is brilliant is coming home from a work dinner or a client at the wrong end of the country or miles of average speed checks in the depth of night. It makes sure I keep the speed safe and legal. Although given a choice I’d have a limiter over cruise control for that.