larian doesn't care about game balance. or they have no idea, how to do that. just look at some of the magic items in the game.
bane on attacked enemy
+2d4 to magic attack rolls
+1d4 psychic damage per magic missile
ray of sickness as cantrip
just to name a few.
they either don't care or have no idea about balancing 5e.
While I agree with you that there are significant balance problems with items/advantage, I think the weapon that applies bane is a really good item.
A.) It is one of the few reasons to encourage players to join the Absolute's cult: to use these cool magic items. Basically all the other "reasons" for joining the cult boil down to "I'm a sadistic person who only cares about hurting others, even at cost to my own safety" which is boring and dumb.
B.) It applies bane to an enemy, which is not terribly overpowered. I'm not sure if it allows a save (it should), if it requires concentration, or how long the effect is (should be for a single attack or until concentration broken)...
