I think this is one place where the lack of epilogues really hurts the game's story. There's actually ways you could play the "Karlach and MC go to Avernus together" ending in a more positive way- it'd still be bittersweet rather than a straight happy ending, but especially with some of the stuff around the House of Hope there's room for some interesting directions you could take it and have a more satisfying ending even if the engine doesn't get fixed. But we don't get any epilogue, so the last thing you see is Karlach returning to the place she's spent the whole game saying she considers a fate worse than death, and that's that.
