Hmmm... I've never heard of a CRA "assistant". I would suppose
that its like a Research Assistant, being entry level, and prepping you to move up. Covance is one of the big 4, maybe 5, CROs (Covance, Quintiles, Parexel, Chiltern, Icon, et al.) IIRC, they started, offering centralized labs and ECG reads, then grew into CRO work.
Stress in research is a different thing. As a CRA/Asst, most of your stress would come from trying to get sites to send you what you need, and complete your reports in a timely fashion. You will work with excellent Coordinators, some incompetent ones, a few who are actively malfeasant or working for a less than ethical PI. Read: glorified baby-sitter.
I've never pursued it, since I have little desire to travel, but I've worked with quite a few CRAs over the years who were very happy, and quite successful, but I can't remember anyone specifically with Covance to refer you to...