I think it rather depends on the car. The most efficient trips I have done in the Merc by a significant margin involve the A34 south from Newbury. This is a dual carriageway with lots of fairly sharp hills that need quite a bit of throttle. I do this with the cruise control on. In this car it seems that it’s more efficient at higher loads than other cars I’ve had, but it still coasts down the other side of the hills.
In some cars, a tiny bit of acceleration causes the instant MPG to plummet. So when it’s not particularly hilly, the slight acceleration and deceleration you end up doing when not using cruise cost you fuel. The Prius was particularly sensitive to this. In my experience in that car, using cruise was more efficient than not unless it was particularly hilly. A38 in Cornwall for example, the car was thrashing itself on those hills at 70mph which clearly wasn’t effective. That’s where mert’s intelligent crusie would have come in handy because the car would have been able to use battery power to get up the hills knowing it would be able to recharge coming down.
Of course, if you are letting your speed drop on hills you are going slower overall, which would have a significant effect in itself.