Are they mil spec? In other words do they operate correctly over a very large temperature range, with large input voltage variance, under high G-forces, in high vibration environments, etc?

I don't know anything about atomic clocks past what I can read on wikipedia but, I can say there is generally a large difference between mil spec ICs and commercial or industrial grade ICs, I would assume this would extend to atomic clock scale devices as well, but just a guess.