I agree that sad and unsatisfying are not the same thing.
Some bittersweet endings can be incredibly poignant, resonant, and satisfying (in their own way). Part of their power comes from our strong wish that things might have turned out differently. As such, people wishing that an ending could've been different doesn't always mean that an ending was poorly done.
I think an ending can feel unsatisfying when it seems obvious that something was left out, either by mistake or due to not having enough time. Other reasons can include a sudden shift in tone, which can have a variety of causes. The writing for a character or mission could have been handed over to a new person who didn't have the same vision, or who didn't have enough context to write material with a matching tone. Or the original writer could've decided on some "surprise" for shock or laugh value, or decided to do a bait-and-switch of some kind, and miscalculated the reaction they would get. (I'm just commenting in a general sense here, rather than suggesting that any of these in particular happened with BG3.)
I feel like having a good degree of openness in the game's ending is broadly good, as it allows people to imagine a lot of different futures. But I think that we generally want the ending for our character and companions to resonate with the themes we've experienced in their story and the choices we have made, to provide a meaningful sense of closure (which even an open ending can have).
