The logician would argue, You only live once should be rewritten as You live only once, with only next to the thing it qualifies, once.
The logician would be unbearably pedantic, but there is a grain of good taste in the pedantry. Writing is often clearer and more elegant when a writer pushes an only or a not next to the thing that it quantifies. In 1962 John F. Kennedy declared, “We choose to go to the moon not because it is easy but because it is hard.” That sounds a lot classier than “We don’t choose to go to the moon because it is easy but because it is hard."
The far more pernicious issue with pangrams, as a means for evaluating typefaces, is how poorly they portray what text actually looks like. Every language has a natural distribution of letters, from most to least common, English famously beginning with the E that accounts for one eighth of what we read, and ending with the Z that appears just once every 1,111 letters. Letter frequencies differ by language and by era — the J is ten times more popular in Dutch than English; biblical English unduly favors the H thanks to archaisms like thou and sayeth — but no language behaves the way pangrams do, with their forced distribution of exotics. Seven of the most visually awkward letters, the W, Y, V, K, X, J, and Z, are among the nine rarest in English, but pangrams force them into every sentence, guaranteeing that every paragraph will be riddled with holes. A typeface designer certainly can’t avoid accounting for these unruly characters, but there’s no reason that they should be disproportionately represented when evaluating how a typeface will perform.