The part of Lady Macbeth is different from any other role Verdi wrote because the composer insisted that the interpreter must have a harsh, ugly voice. By extension, so long as a soprano’s voice is harsh and ugly enough, she is perfect for Lady Macbeth, even if she sings out of tune, lacks agility, has no access to chest voice, sings unintelligible Italian, etc. etc. It’s that ugly voice that’s the sine qua non.