Why is the heat pump can be checked for performance and refrigerant charge more easily in the summer or cooling cycle then in the winter in heating?

In the summer, during the cooling cycle, heat pumps operate under conditions where the temperature difference between indoor and outdoor air is more pronounced, making it easier to measure performance metrics like temperature drop and airflow. Additionally, the refrigerant charge can be assessed more effectively because the system is designed to handle high heat loads, allowing for clearer observations of pressure and temperature readings. In contrast, during winter heating mode, lower outdoor temperatures can complicate measurements and affect the heat pump's efficiency, making performance evaluations less straightforward.