True. According to the GAS (general adaptation syndrome) when the body experiences stress, it will react to the stress and eventually adapt to it. If you always do the same type of cardiovascular exercise, your body will become very adapt to that. Now you will still burn calories doing the exercise but you become very efficient at it. The muscles become well trained for that movement and become very efficient which CAN lead to diminishing returns. For this very reason you must always be applying progression and overload to cardio as well. Also changing the mode of exercise can be a very good choice.