In which scenario is it best to avoid using cruise control?

Avoiding the use of cruise control on steep highways is advisable because the varying grades can require constant adjustments in speed that cruise control is not equipped to handle effectively. When driving uphill, maintaining power and momentum becomes crucial, and engaging cruise control can result in the engine struggling to maintain speed or even downshifting unexpectedly. Conversely, when descending, the vehicle could accelerate beyond a safe speed if cruise control is active. In such scenarios, the driver needs to maintain full control of the vehicle to ensure safe handling of the terrain, making manual speed adjustment more appropriate.

While it might seem reasonable to avoid cruise control in light rain or while carrying a full load, these conditions generally don't pose the same risks as steep terrain, where the dynamics of the vehicle can change rapidly and significantly due to elevation changes. Open expressways usually provide stable driving conditions that are suited for cruise control usage.