You’re misinterpreting it because of the description bugs edit: it’s correct in the Uncanny case, I know the wording doesn’t help but there’s a Rending/Brittle DEBUFF done to mob and a personal Rending BUFF and that’s what Uncanny is, a personal buff which does work wonders; try it on a crusher.
And below is Rending for you and max Rending (Brittle) debuff that got applied by Trauma’s Rending Shockwave, the 2 stack but diminishing over 100% as you can see.
TLDR: Rending personal buff and Brittle(Rending) team-wide debuff are every strong, stack and are very desirable.
As to your comment, Rampage for instance has the same “3 hits” and you can see it working on a dense horde,tighter groups on light swings, the push-attack also has a wider sweep.
P.S. A 5s search on these forums Psyker melee tests & Force Sword blessing discoveries