Avantages
If you are "one of the boys" you should do well. You get to work on pretty neat things. Salaries are OK, not super competitive--you are compared to your peers within your rank, most times this works out OK.
Inconvénients
You will be passed over for promotion if you are not one of the "good old boys". There are lots of actively operating quid pro quo relationships. People are rewarded for kissing up and personal relationships. They do not have a reliable HR that cares about what is reported to them--they often turn the other cheek. Mid-level management runs that place, regardless of what the upper level management thinks or what their mission statements are. Mid-level management routinely encourages unethical practices, esp wrt charging time to government contracts.