Rending/Brittleness causes you to deal a portion of damage as weapon base damage unmodified by armour. source
This is fantastic on a boltgun, since it has such high base damage and allows it to deal respectable damage to carapace armour which it normally deals about 1/4 damage to.
However on other weapons that can also roll rending/brittleness blessings, this is actually a detriment. Combat axes deal bonus damage to flak and unyielding armour types by default - any rending/brittleness % will convert some of that bonus damage into ‘base’ damage, reducing damage dealt.
Take for example a combat axe that deals 700 damage to unyielding on heavy attack, with a base HA damage of 270. In the case of 15% brittleness (1 attack with T3 Thunderous blessing), a heavy attack vs unyielding would instead do (0.15 * 270 + 0.85 * 700) = 635.5 damage, brittleness can also stack up to about 40%, resulting in (0.4 * 270 + 0.6 * 700) = 528 damage at max stacks. I’ve tested this myself against the Reaper in the Meat Grinder and it’s rather underwhelming watching each hit do less and less damage.
Imagine a team using the above combat axe against a beast of nurgle, one person applying brittleness would reduce everyone’s combat axe damage against it. Granted, the opposite is true in the case of weapons that are less effective against unyielding armour but I think this is outweighed by the griefing potential of causing team mates to deal less damage.
In summary, I think that rending/brittleness blessings rolling on weapons that don’t benefit is counterproductive. Rending/brittleness actually reduces the efficacy of anti-armour weapons which it can roll on and in the case of brittleness, has the potential to grief allies by reducing their damage dealt to the affected target. It would feel pretty bad if you spent the resources consecrating a weapon up to transcendent to gain a blessing that not only doesn’t benefit the weapon but also makes it worse.