I did Phase II from January to March, and Phase III from March to May in time for the summer Phase IV (a few years ago). Winter Phase training can be difficult because of the amount of time and energy that goes into surviving the elements. It's tougher training, but sometimes the training suffers when the weather closes in with conditions so severe that you're barely training at all. The extra time it takes for movement in deep snow, etc., can also detrimentaly affect the training experience by forcing the staff to keep traces and tasks tight enough to stay within the allocated training days for each exercise and still accomplish the required number of tasks. So the tactical decision-making may not be as difficult at times as it can be in summer, but the exertion required to accomplish any task, even over shorter distances, can be greater. You'll also find in winter that the teamwork angle gets crucial; sloppy tent/bivouac habits or commitment to tasks such as care of the troops, sentry routines and section duties can have serious consequences.
Once you've survived a winter Phase, the spring training can be a challenge too. Starting a course with attacks across still frozen swamps and ending it wading through (sometimes still icy) hip-deep water presents similar challenges to maintaining health and well-being of self and section-mates while still meeting the demands of training.
But it's all good training. One thing winter Phases give you that all summer training doesn't, is a practical introduction to winter survival and training requirements.