Vantaggi
For those interested politics, NATO Headquarters is the hub for transatlantic security policy. There is currently no other multinational defence organisation as long-lived or successful as NATO. Secondly, both permanent staff and "consultants" (contract employees) are compensated extremely well. Aside from top-level lobbyists, your NATO salary is much higher than what you would be paid for a similar position in the Brussels private sector. And don't forget, the salaries of permanent staff are also tax-fee.
Svantaggi
For those who are interested in directly shaping policy, the scope is limited since it is the national governments of NATO’s member states who hold collective decision-making power. While the International Staff often have more experience on a particular issue compared with ambassadors and their staffs and can influence the policy debate, it is ultimately the national capitals who will make the decision. In other words, the International Staff might prepare the background paper and then watch from the sidelines. If you want a seat at the table to be directly involved, then you should work for your country’s foreign ministry or defence ministry. Another downside is the lack of promotion opportunities. You will stay in your position and will move up only if you apply and are selected for another position. This might happen if your boss leaves, but turnover is low and most of the high positions are reserved for outside people sent from the national capitals. You might have a greater chance if part of the International Staff is going through one of their periodic organisational restructurings which requires personnel shifting. Related to the previous point that there is no expectation of career advancement, you will not have any professional development or training. Aside from informal coaching or networking, new hires are experienced professionals and are expected to know how to do the work. They do not grow someone for a job. Once you've settled into your job and are performing satisfactorily, that's great! Keep it up and if there's enough funding, they'll continue renewing your 6 or 12-month contract. If you have a permanent position in the International Staff, you can stay in that position until retirement.