The problems they're solving are pretty constant, but they go very deep technically for those who are interested. There's a very long learning curve compared to most private sector jobs, but you can power through it in proportion to your personal ambitions.
Downsides: (1) You're a political punching bag for 50% of the candidates in each federal election, except in years where military power is on the electorates' minds. (2) Mediocre pay. (3) Soul-crushing bureaucracy. (4) It's the only job I've ever had where the employer has missed payroll.
Other pros: (1) Working with the same folks for many years can be nice. (2) Within limits, national defense is really important. If you want it done well, this is an opportunity to pitch in.
Caveat: invading Greenland or Panama isn't what I'd call "national defense". But the learning curve / hiring process are too cumbersome to quit and rejoin every 4 years depending on politics. I know of no good solution to this one.