I don't play melee heavy character, so finding a Pickman's knife (don't you just love HP Lovecraft references in Fallout?) that does nice 25 bleed damage. Now, is this damage additive with every hit - I mean if I hit 3 times quickly, will it drain 75 damage on top of normal instant one? Also, is such Damage over Time affected by sneak crit/bonus?
- Wounding stacks, and is particularly effective because of the lack of bleed resistance. Even enemies that seemingly wouldn't be susceptible to bleed effects, such as robots, take bleed damage per hit. This makes wounding especially useful on fast or multi-shot weapons where multiple applications of the effect can be applied quickly.
- Poisoner's does not stack, and is mostly ineffective due to the long duration of the effect (10 seconds) and poison resistance/immunity of some wasteland creatures.
- Incendiary does not stack. It can, however, cause flammable environment to also catch fire, such as oil spills.
Other weapon effects (freezing, plasma-infused, explosive, etc.) are direct damage modifiers, and therefore do not have any damage over time effects.
I don't think these effects are altered by a sneak/critical, or other bonuses. I cannot find anything reliable documented that states one way or the other. But, based on my experience, and some of the verbiage on the linked sources, it seems these effects are applied separate from the base gun damage, meaning these 3 do not benefit from perks, sneak/criticals, etc.
Damage over time stacks. I fought a Legendary Super Mutant and noticed my health draining faster when he hit my several times in succession and slower when he only hit me once. I modded his Wounding Institute Rifle to be an automatic and now things melt.