if a monster is immune to an element, it has 100% or more resistance for that element. having 99% resistance makes it no longer immune, and at that point the -x% enemy resistances from items and skill that don't break immunities start working.
most monsters have 0% resistances to elemenents they are not immune to. some very rare monsters have more than 0%. since negative resistances cap at -100% iirc, having more pierce than that is useless if the monster has 0% resistances. only if the monster/boss has more than 0%, is it useful to have more than -100% pierce.
in case of heroic bosses, if their res drops from 100% (immune) to 99%, they are no longer immune. now you can stack -99% enemy res to get the heroic boss resistance to 0%, and then another -100% pierce on top of that to get them to -100% res like the other monsters - in total -199% pierce for heroics.