The critical stresses on the balls will be higher than on the races, due to the balls' smaller net radius. Hence, everything being equal (e.g. hardness), the balls would exhibit fatigue failure first (flaking of the surface). For maximum time between overhauls, there is a case then, for making the balls even harder than the races.

I've seen some cases of the balls failing in fatigue, but most of the messed up bearings (at least good quality ones) I've seen, have been messed up due to lack of lube, or water contamination, or both.
The only races I've seen with fatigue issues are hub cones of the Normandy/Atom variety; and this seems to be pretty consistent in my experience, and reading on this forum.

So as a PSA, if you have any french hubs with miles on them that you haven't checked, take a look, and be prepared for an overhaul with new cones (and new balls as a cheap precaution).
That said, so far, all the french hub cups I've looked at have been fine, for whatever reason.
Hmmn, come to think of it, maybe I should take some of my own advice...