Stress
The stress, while not horrendous, is still rather high. The deadlines are tight, the money placed behind an engineer's project is astronomical, and the fact that someone is going through your garbage every now and then to be sure you're loyal to your employer can all add up. Government oversight committees are notorious for wanting quick results and do not deal well with disappointment.
This isn't even to mention the fact that the things a defense engineer develops will likely lead to people dying. It may be a lone terrorist or it could be an entire village that harbors the enemy with "acceptable collateral damage," but chances are pretty good that someone somewhere will bite the big one because of an engineer's work.