It's mostly anecdotes and personal experience. I think that data on salaries, etc, is mostly public too, so you can probably see for yourself that they aren't competitive with SV or Wall St.
Also, to say they are mostly corrupt or incompetent seems possibly too much of a broad stroke that is based on anecdotes. I don’t believe Snowden, for instance, ever suggested either of those are true — did he?
Finally, if this is your assessment, your standard may be unusually high. Maybe your peers, many not quite as good as you, are actually quite skilled? Just a hunch, could be wrong.
Aside from some research that the NSA has contributed to humanity, they have done little good based on the evidence I've seen.