Not entirely sure what you mean, but if i understand you correctly, what you say is also not true for V2.
Damage bonuses for enemy “class“ and “health type“ do stack though.
Damage bonuses in V2 and Darktide are actually pretty much identical in that regard.
Anything that you can hit, belongs to a class and a health type.
V2 enemy classes (factions): chaos, skaven
V2 health types: infantry, berserker, armored, monster
Darktide also has perks with damage bonuses vs enemy classes: elite, specialist, groaners and poxwalkers.
In addition to perks with damage bonuses against enemy health types: infested, unarmored, maniac, flak armor, unyielding, carapace armor.
V2 enemy is armored skaven? Then armor and skaven bonuses apply.
Darktide enemy is flak armored elite? Then flak armor and elite bonuses apply.
Not complex at all.
Dots are always non crit, no weakspot and apply to the “main health type” of each enemy, independent of how they are applied.
If you apply a dot to any enemy in the meatgrinder, the main health type of the enemy is displayed above their head, while the dot is ticking away. That is what the dot is dealing damage to.
If you apply a bleed with a knife for example, a different health type might pop up initially, based on where your knife strike hit the target (for reapers, it can be unyielding, flak or carapace depending on where you hit).
But independent of what health type you initially hit, the main health type of the target (unyielding for reapers) will soon show up, indicating the health type that the dot is attacking.