The radiant buff gives weapons that don't have intrinsic champion effects anti-barrier and the ability to shoot through various enemy shields, but currently it does not provide that effect to wardens law even when you have zero artifact mods active and from what I can tell it hasn't provided this anti-barrier effect for more than 5 months now. With this being the nightfall weapon for a week with double nightfall rewards many player are going to get this gun, and find it just can't pierce shields or barriers as it should when the radiant buff is applied.