Avantages
Great job security, especially if you have a clearance. I started my career with Lockheed and I'm glad that I did. The engineering is rigid and process driven, but that can be beneficial in some ways when you're a young engineer
Inconvénients
Engineering environment is incredibly stale. If you work classified programs then expect to work with old technology or build everything in-house. Any software libraries you want to use have to vetted by the security team (takes months or years). The longer you're in defense contracting, the more valuable you are to defense, but you become useless anywhere else because your skills and tech familiarity will slowly degrade. They rarely fire people, so find plenty of underachievers that are just productive enough to keep their job.