Unknown alloys is the trick. Before the switch to zinc, wheel weight alloy varied with the region of the country you lived in, with the antimony varying about 2:1. Having a way to check hardness has always been useful in case you needed to make adjustments. But the hardness testers made for hand loaders have never been wildly expensive. Current prices at LBT and Midway are:
LBT $125 - Reads in BHN directly.
Lee $69 - Makes indentation you measure and look up BHN on a table.
Saeco $193 - Makes and measures indentation on an arbitrary scale, for which reading you then look up BHN equivalence on a table.