Not sure if it's the same for others, but cruise control on my 2012 won't engage or decrease below 25mph (pretty normal) but I've found a way to get it down to 20 consistently, sometimes lower.
First, set cruise near 25 and then tap "Set -" until it's settled into it's lowest setting. Once that's done press and hold "Set -", which will slow the car further without disengaging cruise. After dropping further, release "Set -" and quickly tap "Set +" once.
This will set cruise again at this new lower speed. It does seem that if you pull the cancel/resume paddle it will be lost entirely, as if cruise has no saved speed setting.