Hi Kravyshank.
It really depends on the downhill slope you are travelling & the speed you are doing. I live in a mountainous high altitude location with various hills of many shapes & steepness. My experience with cruise controls of various designs is that on incline or decline, they will have issues & not be to our expectation. It's not the same consistency as what you'll get driving in relatively flat highways.
There are just 2 situation I simply turn-off the cruise control. 1) Driving in the snow 2) Encountering major uphill/downhill situations.
At certain steep angle, even reasonable automatic downshifting to employ engine brake will not be consistent or even reliable to maintain constant speed going down - if programmed on higher speeds, and while on steeper downhill. Carrying load affects this too.
On a decline of 6% while doing 45 mph; I found that the lowest CVT downshift range in the JX35 is down to the equivalent of 3 or 4 range, but it would not do less than this. I am not sure if the ICC would even engage the brakes. It did one time, but not because of the downhill speed, but because it "sensed" a slower car located downhill ahead of mine.
My experience is the same as yours that the system will simply allow an overshoot beyond programmed speed & indicate a blinking velocity on the dash. However, in fairness, this only happens in steep downhill on a full heavier load at higher speeds.
At lighter loads, travelling on a much benign slope, and with lowered programmed speed (< 45 mph)... it still performs as expected.