the barbarian passive seems it's not stacking with other sources of +accuracy buffs? e.g. devotions for the faithful (priest). that seems odd, anyone else experience the same? i'm playing with the deadfire tweaks mods that changed accurate carnage to +8 from +5. maybe the mod bugged it up somehow?

Maybe. Usually Accurate Carnage is a passive ability and those stack with everything. Maybe your mod doesn't flag it correctly or even the original has this bug.

I confirmed that it's stacking with Paladin's Zealous Focus (+5 Accuracy). It's either the mod, or a specific issue with Devotions for the Faithful.