having just equiped a sunder charm on my light sorc which is currently running lvl 26 nova /static , i find i still have no hpe of killing a pack of light beatles in act 2 (looking for Duriel tomb). I read elsewhere inthe forums that sunder charms only drop the mobs res to 95 % … so for the sake of finding the extra resists to cater for the charm is it worth having them? am i missing something? is it the same for fire/pois/cold etc?. feeling a tad frustrated
The fact is they should have never nerfed them, now they are only viable with Cold Sorc and Infinity is still your go-to.
But even in their original state you would get most value out of them when you get Infinity anyways, and that value would be breaking the very limited Immunity that Infinity does not break.
There needs to be class balancing.
The charms are not a stand-alone thing.
The charms only function well if you also use conviction via the infinity runeword (impossibly rare runes), lower resist curse via npc purchased wand on weapon swap (annoyingly clunky), and/or unique jewels (also very rare). Lightning immunity would not be broken at all before the charms were introduced, unless using more than one source of these things.
The conviction aura and lower resist curse could also be achieved through group play online 
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Otherwise: yes, like you noticed, you are only dealing 5% damage to something with a ton of life 
If you had a crescent moon it would be -35% , and a griffon with a facet would be anywhere from another -17 to -25. However you get it, you absolutely must run negative enemy res to properly damage sundered immunes. It will also help against non immunes.