Hi!
Thank you for taking the time to report bugs, its very apricated and help us make the game better!
First of all. In Darktide a armor in itself does not apply a armor modifier. Its just notates what kind of armor a enemy have. Instead each attack have the armor modifier in it. That means that different attacks will get different amount of benefits from armor penetrating buffs like rending or brittleness.
(@Pygex is essentially correct)
Instead of armor having a 0.5 armor mod with all attacks, we specify in each attack what armor mod that attack have. Thunderhammers light atacks have ~0.5 against flack armor while its heavy attacks have ~1.0, and so on.
In this example brain burst penetrate all armor except carapace armor (i.e it have a armor mod of 1.0 against all armor except carapace armor). Hence it the only benefit brain burst gets with brittleness is against carapace armor.
Brittleness and rending is suppose to represent that either the armor is broken or that your attack penetrate the armor. If your attack already does this by itself it can’t get any benefit from it. If all armor is stripped already you don’t get the benefit of stripping it even more.
(For the case of brain burst the reasoning I think is that only the thick carapace armor can act as a shield against a psyker penetrating a brain to make it pop)
Do your test again but use a physical melee weapon like a combatsword and you should see a big difference.
Regarding @CommanderJ s question about brittleness for trauma staffs @1Cake is correct. The trauma staff itself will not gain the benefit of the brittleness since its attacks have good armor mods against all armor types but allies will benefit. You will also get benefit when being forces to switch to melee or using the trauma staffs main attack which have very bad armor penetration against monsters and carapace armor.
But it also depends on what weapons your teammates are using. Are you all psykers that only brainburst? Then not so much. An ogryn and a veteran that uses shotguns? Yes, they will benefit a lot.
Not all blessings are meant to buff the current weapon or attack you are using. In the case of the trauma staff I think you can see a use case of you charging the trauma staff against a crusher or monster a couple of times until you peril is maxed then switch to a Force Sword with the Unstable Power blessing (power bonus with higher peril) to finish it of as your peril dissipates. With the rending you otherwise weak against carapace armor Force Sword is now doing damage against the target.
I hope this helps and clear things up!
Best regards! / Guen