I have noticed this on my 04 F150. At the end of a long grade is really bad, very slow to resond at bottom of hill and maintain the speed setpoint. It is really annoying if there is a hill going up at the bottom, can lose 5-10 mph.
As for the Mustang I have not experienced the problem, although I only have a small number of hwy miles and alot of that is without the cruise.