EMP resistance. This isn't exactly a practical justification for most uses, of course.

Would they be resistant to EMP, surely you've got lots of possibility for induced currents that will warp/melt fine components sufficiently to damage the watch?

A sundial is what you need.

I feel like most EMP damage on small electronics comes from blowing out small in-chip wires and transistors. I wonder if an EMP could induce enough current to overheat those little (but large by chip scale) silicon wires.