Hey there, a weapon's intrinsic anti-champion effect will always override one from a mod. Eriana's Vow will never gain the benefit of Unstoppable Hand Cannon, so that part is working as intended.
Would you be able to provide a video of it not working against a Barrier Champion's shield?
English
-
It is stunning, it's just taking 2-3 bullets when it normally takes 1-2.