What do you think is the most difficult part about using aperture and shutter speed?

Aperture determines the depth of field of the photograph. However, different lenses have different depths of field for the same aperture setting. The depth of field also varies based on the focusing distance, or the distance of the subject from the focal plane. So, it takes time to get used to a new lens to get the appropriate or expected depth of field for photographs. In the case of shutter speed, it takes time to get used to the optimum shutter speed to be used in a setting based the lighting available so that there is no shake visible in the image. The ISO setting also affects the shutter speed to be used for optimum exposure. Becoming skilled enough to use the right combination of aperture setting, ISO setting and shutter speed for the right exposure and to get the correct depth of field without causing visible shake in the image is a skill that needs to be perfected with practice.