The downside to `None` is that it's one more thing to check for and makes object arrays an even weirder edge case.  (Incidentally, Decimal does have its own non-float NaN which throws a whole different wrench in the works. `np.sign(Decimal('NaN'))` is going to raise an error no matter what.)

A float (or numpy) NaN makes more sense to return for mixed datatypes than None does, in my opinion. At least then one can use `isfinite`, etc to check while `np.isfinite(None)` will raise an error.  Furthermore, if there's at least one floating point NaN in the object array, getting a float NaN out makes sense.

Just my $0.02, anyway.