Turn that thinking around, the best economy you'll get with cruise control is on flat roads.
In anything but a flat road, a skilled driver will beat the machine every time, the jack rabbit
will be too interested in overtaking and braking behind the next car up the road.

Hybrids are interesting because varying the vehicle's speed enables the electric side of the car to do
its stuff with regen brakes and releasing energy on acceleration so the car gives better economy
in variable speed situations.