I think CC would work on flat terrain and with no traffic.

CC cant' keep a traffic buffer, anticipate changing conditions.
One of my traffic pet peeves is being behind another car with CC on driving the posted limit. In that instance I need to create a huge buffer, or suffer pulling the next hill with needlessly low MPG for a much longer time.

I've noticed in my own car that the CC will often "fish" for the best acceleration point: Suppose I'm on a flat area holding 70MPG and approach a hill and don't want to go slower. Suppose I have to drop my MPG to 56MPG to overtake this hill. It's a smooth transition from 70 to 56 to 70 again.

With CC there is often a large, unnessessary swing in MPG. For example:
If CC tackles the same hill it will likely begin around 70MPG until the beginning of the hill.
As the hill begins it has no idea how much load there is going to be so for a moment it drops down below 40 and might even use Assist. Then rises above 60 before settlling to around 52-54. It doesn't try to pinpoint max FE for the given situation: It just knows that 52-54MPG holds the speed for that load.
That's a significant drop in MPG vs. manual.
If I approach another hill the above is repeated.
Sometimes these fishing swings are larger than other times.
If I roll down the hill on the other side I still get the same MPG manual vs CC.

This may explain why my CC can get more than 10 less MPG over a given trip.

The only time I'll briefly switch it on is to either stretch my Right leg or take my Right shoe off for a better feel. Then it's off again.

I'm not sure about +75MPH as I can't remember the last time I had it that fast...