Activation base is in Stamina and values vary with fatigue%.
The character strikes twice in rapid succession when activating this talent, both for normal combat damage. It is thus much stronger than expected. The armour debuff is not applied until after both hits, however.
Sunder Armor's applied penalty does not stack with itself.
Enemy AI uses this talent based on chestpiece type only. If it's medium, heavy, or massive, the AI will try to Sunder Armor. The actual armor rating, as well as helmet, gloves, and boots type are not taken into account.