I don't do too many long car journeys, but I recently undertook a 300+ mile round trip so got a chance to try out the cruise control function on my car. It's not the first long run I've done in my car but for whatever reason I've never bothered before.
Each system is different and it took a bit of time to figure out what I was doing but I soon got the hang of it. On the return part of the journey I didn't bother with it, instead focusing on getting better MPG (the old game of trying to get the figure on the car computer as high as possible). This meant going slower when heading uphill and picking up speed when heading down. I was amazed at the difference.
Cruise control is a fairly blunt instrument, you set the speed and system will try and maintain it no matter what. You head uphill and it applies more power to overcome the incline, you go downhill and it won't coast, it'll brake to get you back down to the set speed. If you have to disengage it for any reason and then re-apply it'll usually floor it to get back up to speed.
Manufacturers already offer adaptive/autonomous cruise control systems to monitor traffic in front and slow down before speeding up again once in the clear. How about an economy mode for cruise control?
It's not as simple as just maintaining a constant speed, but it'd be easy enough to detect an incline and not apply additional power, or to let the car cruise down a hill with a degree of leniency for going over the required speed (this is probably the most contentious part, especially if speeding tickets are involved). Add it to the systems that can monitor traffic in front and slowly drop the speed as they approach a slower vehicle and you should see an improvement in MPG compared to traditional cruise control.