As far as I understood, the "karmic dice" setting should "smooth things out". As there is no ingame description, I expected that this just means that the overall roll distribution stays the same, but it tries to break lucky/unlucky streaks. However, it seems to consistently make enemies hit more than they should, effectively working as a significant AC penalty.
Explanation posted on Reddit: 
https://www.reddit.com/r/BaldursGate3/comments/zwqaem/psa_having_the_karmic_dice_setting_turned_on/Data: 
https://docs.google.com/spreadsheet...JpkP9anhPshxjVinoXwKdYByYhQkhIxm/pubhtmlSuggestions made by the community:
-  Disable karmic dice by default
-  Add more finegrained settings (karmic dice only out of combat, or both)
-  Add more explanation what the effect of this setting is
-  Rework the karmic dice rolls
As far as the data indicates, this setting mainly makes the fights faster and more unforgiving, which is especially punishing for newer players.