Pros
Ph.D. students are well paid in Belgium in general, and my experience was having a lot of freedom. As opposed to the Ph.D. students at a university, I could focus 100% on research. Colleagues are generally nice and down to earth. If you can arrange it yourself, you are free to go places (conferences, research stays, ...). Plenty of other Ph.D. students/interns/master students etc around to make friends with. You can have plenty of opportunities to develop yourself with courses/summer schools, ... Strong safety culture which is important when you work with radioactive materials. Recent much needed upgrades in infrastructure and equipment. Some exciting projects going on and generally there is an openness for some fundamental science. Secretaries, at least in the institute where I was, were angels.
Cons
Downsides do not pertain only to me or Ph.D. students, but are general observations about the culture. Specific to the Ph.D.: Ridiculously over the top hiring procedure for Ph.D.'s, including way to much paperwork and a highly stressful presentation in front of a whole panel of high ranking professors. This experience is repeated twice during the Ph.D. in the form of Ph.D. days, where the only goal is academic hazing. The Ph.D. experience depends highly on the group you end up in, which you don't know much about a priori. Often mentors know very little about the Ph.D. topic and you will be completely on your own after a few months. In other cases, Ph.D. students are abused as somewhat cheap but mostly flexible technicians to churn out data for repetitive qualification tests. The goals set forth by the university will often clash with what SCK wants out of the project. Specific about the environment: middle of nowhere place without any public transport connections. Used to be a nice campus populated with trees, but for fire safety all were cut down. For some types of research very limited tools available so you need to seek ways to do work externally. Specific about the culture: few staff scientists are actually doing science. They spend 80% of their time on project management and bureaucratic matters. Their only contribution to science is through their students. There is a terrible meeting culture, with tons of meetings being held and very little decided. Rarely will there be an agenda before a meeting. There is a strong divide between technicians and scientists: time when they start (scientists ~9:00, technicians as early as possible), where and when they eat lunch, time when they end. Many technicians are purposely unhelpful, especially to Ph.D. students, though there are a few hidden gems. Everything is painfully slow and bureaucratic: to get something done when you rely on someone else is a nightmare. Bureaucracy, will be invoked as a reason to not do something when possible. Everyone is always "super busy", yet you see very little activity going on. Getting something made in the workshop requires constant in person visits because higher ranking individuals always skip the queue. Forget about getting something done from the purchase department. Higher management focusses on projects that can bring in industry money or that can lead to patents and products rather than publications and discoveries. Being competent is a downside, since you get more responsibility for no reward. From what I've seen, promotion depends more on how cosy you were with the higher ups than competence. Very linear and quasi-static promotional ladder biased towards years of service versus performance and intentionally crafted so by unions. IT infrastructure is an absolute nightmare if you need to work with anything more complex than microsoft office.