My guess would be that in the USAF the failure rate over the last several years for 1- through 3-dan (these are the only dan ranks for which they test) would be close to zero. In fact, I wouldn't be surprised if it is exactly zero, though I would be very surprised if it was even as high as 5%.
The last time I watched dan tests at a USAF seminar, six people failed their test. Four shodan tests and two nidan tests failed.
I don't remember the exact number of tests, but that six was probably around 25% of the tests that day.
My sensei also told me about one time where EVERYONE failed at a dan test.
I have watched a lot of dan tests in the USAF and I have seen many people fail. Next year, I will test for shodan and you can bet I will be 150% ready before my sensei lets me test.