Unless your concentration is intense and unlimited, your mpg will be better on standard CC (whose concentration is intense and unlimited).
In the old days with the ultralight and underpowered subcompacts I used to drive (none had CC), it was easy to discern a slight engine tone change and change the pedal setting before the speedo changed. I was pretty focused in my youth, in fact I think under those circumstances I was able to hold a set speed better than modern CC all day long. No way could I manage that in the Niro.
But I must say I wonder why modern CC cannot hold a speed on hills. Millions of inputs per second speed and load, electronic control, and it will still drop or exceed by a mph commonly and slightly more on certain hills. I suspect there is no inclinometer or accelerometer installed but for the buck it would cost, I'd think programming could be far better.
2018 Kia LX HEV Metal Stream with Advanced Tech