Every project is unique and hence complexity differs from project to project, and also it depends on the skill level of the project manager and also the maturity of the organization, the support it gets from the senior management, and other such multiple factors.
However, at a thousand feet level, if at all I have to classify a knowledge area over the other, then Risk management would be tricky.
Especially considering how you are going to quantify the risks, having an equal focus on both opportunities as well as threats and it is an underlying principle for the whole project.