I have a Ford Escape Hybrid.
Driving manually gives 10% better MPG than Cruise Control in every condition other than perfectly flat, and there are very few perfectly flat roads. On perfectly flat roads, cruise control matches manual driving, but never beats it.

I have a Honda Insight.
Driving manually gives 20% better MPG than Cruise Control. This helps explain why none of the Insights sold came with CC as a factory option. Every owner with it, has added after market cruise control. Honda did not offer it, and it looks like for good reason.

Part of it is, my human foot lets speed fluctuate a little. Driving with load constant is much better than driving with speed constant. Also, I can open the throttle gradually on hills, the CC tends to floor it no matter the size of the hill.

Now, both of these are cars with really small engines.

With large engines, cruise may help more than hurt.