Studying Hume-Rothery rule for substitutional solid solution, i have found the so-called "relative valency factor" stating that, for other conditions being favorable for substitutional solid solution, low valency metal can dissolve more high valency-metal, but high valency metal dissolves less low valency metals. Why?

an answer was- metallic crystal structure stability is more sensitive to valence electron density decrement than increment. but why does that happen? one answer pointed out to Brillouin zones of metal, but I am not comfortable with notion of "Electron filling Brillouin zone". Since Electronic band structure can be explained without intermediation of Brillouin zone, at least in freshers undergraduate level. can anyone explain it to me in simpler terms? the paper about brillouin zone explanation is attached