I have noticed the exact same thing. By the time I am nearing the top of the hill, I have overshot the set speed by 2-3 miles per hour and on the other side by 5-8 MPH, then at the bottom I am under by 3-4 MPH. Flat land it is great, but that is not a real test of cruise control.

I had to leave work and head back into Houston for some personal business, but on my trip to work this moring I drove without cruise and got about 40 MPG indicated on my nav, on my second drive to work with cruise I got about 37 MPG. The temp in the morning was about 45 and the temp this afternoon was about 65, and all other conditions were fairly similar.

IMPO, the cruise should interact with the hybrid computer more. When the vehicle speed decreases, it must be from an incline, and thus the motors should assist more, and if the speed is increasing, it must be from a decline and the motors should regain some energy to keep the car at target speed. They could easily dampen the power needs of the ICE if they used this form of stratagey and get much better FE since the engine could be left in an efficient burn range and use the motors to adjust power output.