Sometimes it is not what the impurity is, or how much is present, but rather the amount of variability that is introduced by the variability in manufacturing a 'cheap' household product compared to a photo grade chemical.
As an example, Detol contains bromide. Today, most papers are high in chloride and some are pure chloride (or nearly so). Any variability in bromide purity or any iodide salt in any of the chemicals causes a big swing in speed and contrast of chloride papers. Iodide in some salts in developers for films changes the edge effects because iodide is a 'buffer' for edge effects.
So, some hidden or difficult to measure effects may vary and qualitative or eyeball tests will not show it. And, BTW, this becomes even more critical in color.