I have found that volume units, such as cup, are fine for measuring liquid ingredients, but generally not a good way to measure certain dry ingredients, because of the differences in how they can be packed. For example, one cup of flour or sugar can vary significantly depending on how tightly it's packed in. Beans don't have the packing issue, but it's hard to get them level. So again, volume isn't ideal. That's why I prefer to use mass. I have a food scale, and if a recipe calls for, e.g., a certain number of cups of flour, I use the volume-to-mass conversion factor on the side of the flour package to measure it out in grams, which is a mass unit. That avoids the issue of packing.