Pros
Mission-based work advocating for workers means that you will work with smart and kind people. Coworkers are some of the best people I've ever worked with. Depending on department, the work-life balance can be good. Benefits such as pension (you contribute) and health insurance (don't contribute to premium) are top-notch. Could improve number of vacation days but it is better than some private sector (the greedy ones) employers.
Cons
VERY limited opportunity for growth and advancement unless your supervisor favors you and you advocate for yourself on a regular basis. In my department, some new hires were placed at mid-level positions (job description says only 5 years experience for such level) and both new hires had between 15-20 years of experience. I believe these new hires applied for senior level positions but were downgraded when hired. Insulting to say the least.