In general, sundering mods are bad. They provide very little benefit in comparison with other mods. First of all, there are only two classes that can even use a sundering mod: the ranger and the warrior. So, in order to explain how other mods can be better for you, consider this:
Q: Why do you use a sundering mod on a warrior or ranger?
A: To increase your DPS, of course.
Now, if you wanted to take this from a technical standpoint, you can. Since an axe or sword attaks a 1.33 second attack rate. With a perfect sundering modification, a perfect +35% ideal axe will only increase the average dps 0.293 (against an unbuffed 60 armor target). For that extra 0.293 dps, you're spending a large sum of gold for that modification.
On the other hand, you can equip zealous modification for superior energy management. With 1 pip of energy regeneration and with 1 hit for every 1.33 seconds, you'll have around 1.08 energy per second. So, if you used that ~1 eps or so to fuel, say, frenzy, you would have a 33% damage increase, taking your DPS up to 32.6.
Now, assume, you didn't want to use your energy skills. You want to run some sort of warrior loaded up solely on adrenaline skills. First of all, that's a bad idea - using frenzy can easily help your dps better than a sundering. So, compare this to other mods:
Using an elemental mod, you can negate other warriors' +20 armor bonuses, which turns that 100 armor to 80, so you'll be causing another 25% damage.
If you didn't want to do that, saying you won't be able to kill rangers well, then you can also use a vampiric weapon. A vamp will increase your dps to 25.91, with five damage per hit armor-ignoring.
Finally, people think that their sundering mod will help their damage against a higher-armored target, such as a warrior. Well, so what? With your 10% sundering mod, your DPS will be increased by just .207 to just 11.127. A vampiric mod, however, will cause 10.92 DPS plus 3.76 from the vampiric mod, for a total of 14.68 DPS. Finally, using an elemental mod against a warrior target will cause your DPS to be 15.44 against that warrior. Your zealous mod will give you 16.3 DPS. Looking at those numbers, even against higher armored targets, sundering mods can provide less benefits than any other mods, except perhaps furious and elemental (in the case that it's not against a warrior.
When you look at these numbers, you come up with this:
22.133 DPS against a 60 armor target with a 10% sundering mod.
21.84 DPS against a 60 armor target with a zealous mod, but you gain around 1 energy per second (including energy regen), and frenzy can be used to kick up your DPS to 32.6. In addition, using frenzy will increase your DPS further.
Even an elemental damage mod will increase your damage better, with a 25% bonus damage on a warrior.
A vampiric weapon will give you -1 health degeneration but will increase your DPS to 25.91, but will give you 5 health per hit.
Finally, sundering is the most expensive of them all. A sundering mod will cost tens of plat, while a zealous mod will be a could plat at most, and elemental mods are mere hundreds. A perfect vampiric may cost 20-30 plats or so (not sure; it's been months since I last played pve) at the most.
Hopefully this post will help the mistaken GW players that are intent on using their sundering mod. All numbers were taken using the GW damage calculator and the built-in computer calculator.
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