Lack of security. You have to reapply for a job every few months. If you cannot get on a project you will be out.
Lack of control. Limited choice of projects to work on. Very rigid scope and timescales for the ones you do.
Poor lifestyle. You will be away from home all week. Long hours. Lack of friends/social life.
Stress. You are smart and work long hours but to justify that day rate you need to be smarter and working longer hours.