Regarding Karlach's power level, I think that being from the blood war isn't in itself over the top. I think the concept of a tiefling who got sold into fighting in the blood war is genuinely pretty interesting and an acceptable backstory. It's when Larian started adding to that, saying she was Zariel's favored warrior, some elite, well-known warrior of Avernus and when they put the infernal engine in her chest that was when it got to be too much. They simply lacked restraint. She could have just been an unlucky warrior who struggled and survived by the skin of her teeth in a meatgrinder she shouldn't have had any business surviving in. But Larian simply had to just keep piling on, as they did with everything else.
