Pros
Good place to build expertise if you are in the right team, offers a lot of different great training opportunities. They compensate for overtime and respect work-life balance. There are a lot of young people from all across Europe so it's a good place to make friends if you're new in Brussels. I think most people make some very meaningful friendships with their coworkers which despite all the issues (see below under Cons) is a big deal.
Cons
The base salaries are quite modest unless you work a lot of overtime. The opportunities to advance up the ladder are few unless you are Belgian (preferably French speaker). This is one of the very few successful companies of French speaking Belgium, therefore promotions mostly only go to Belgians who then clique up to make sure no non-Belgians cause any disturbance in the force. Overall in your daily work not much room for individual initiatives, do the job as per the SOP and shut up, it feels a lot like working at an assembly line in a factory. Overall it's extremely bureaucratic, I remember once to get reimbursed for some minor expense of literally a couple of euros I needed to follow a manual with 29 steps, I accidently skipped one step and had to start all over again.