I assume you are referring to working with a contract house or maybe selling yourself directly to potential employers.
I have never personally worked as a sub-contractor with a contract house however I have known many over the years. I do and have worked a contract trainer and consultant for many-many years.
What I do know about contract designer engineers and engineers is that you should prepared for travel to various locations, have diverse CAD, CAM, and CAE software skills and be willing to take on the less than optimal job tasks.
Moreover, know that these jobs can last for short duration and sometimes people transition into permanent positions.