are those long, cruise control drives bad for cars

Cruise control exists for a reason; long drives. Right? I read in my manual that one should not stay at the same speed for long periods of time (I’m assuming this is when the car is NOT on cruise control). Does that mean staying at 1 speed for long periods of time is OK if one is using cruise control? I’d assume so. I actually find it difficult to stay at one steady speed when not on cruise control as speeds does vary.