I think there are two examples of very good bittersweet endings in the game. Karlach's choices at the end are hard to deal with. The dissatisfaction with her if more about the story getting to that point. She's a lovable fun character but we do very little for her and see very little character growth.
The other ending is one of the Halsin endings, the one where you choose to become a mindflayer.
He still loves you, worries about your safety and you can join him in his new venture. The bittersweet part? We know from the Emperor's letter to Ansur, plus dialog with Omellum, that mindflayers don't have emotions.
It's brutal. And that's okay. It's an actual ending, not "bye, come visit sometime."
