Avantages
The culture at ORNL varies widely by group/section, but the toxic work environment created by senior leadership is pervasive. Depending on the group that you are in, you may or may not be isolated from the worst of it, but it is always in the background. There are many good people and researchers at ORNL, but many of them have left or are leaving. ORNL looks good on a resume, but so does working at another less toxic national lab.
Inconvénients
Take a good look at the negative reviews here on glassdoor (especially those by people who have been at the lab for a while) and know that if you are in any sort of leadership role (project lead/group lead or higher), you will be dealing with those issues on a daily basis, and even if you are not in a leadership role, those issues will randomly cause chaos in your day-to-day work like stray bullets. ORNL has steadily become more and more toxic since the new senior leadership took over in 2018. It wasn't always like this. The current senior leadership is more interested in personal glory and building legacy than actually creating an environment conducive to research, and that's the culture/attitude that's rewarded at the lab. Lab-wide decisions are made by senior leadership on a whim, and whenever things go poorly, blame is placed on the researchers. Most if not all of the changes made by senior leadership have made it more difficult to conduct meaningful research effectively on a day-to-day basis -- things like arbitrary lab-wide restructuring of every group where people had to reapply for their own jobs, removal of 20% free time for postdocs to pursue their own research and using that money to fund more meaningless middle management roles, a ban on hiring anyone graduating from University of Tennessee (that was later rescinded after senior leadership insisted that everyone had misinterpreted their directions), and many countless more lab-wide one-size-fits-all policies that feel like death by a thousand cuts. By the time I left, I never felt so unsupported in my research and never had so little work life balance in my life.