I understand the passion, I have whole sections of the game and story I personally think was poorly done and even outright hate. Those feelings are cool. It is testiement to how good the writing and character acting is that people have as much passion as they do about many of the NPCs.
Well, I mean, yeah! In the end that's entire point, I think. We all here liked the character so much that not being able to give her at least a happy-ish ending (granted, you can interpret the Avernus option differently, but clearly not everyone saw it as hopeful - I think the fact that all of us see fiction through the prism of our own experiences and convictions plays a big role here for sure) was unbearable enough to start looking for signs that we missed something. And as things stand now, we didn't - but that's where the short quest and the gap in content compared to other origins becomes glaring, 'cause that's the only thing we find when looking for solutions.
In some way all such criticism is "arm chair quarterbacking", I think, especially since none of us (I think?) are devs ourselves. I worked in another similar story-creating industry, I've sat through my fair share of seeing people on social media rage about a plot twist I knew my colleagues put a lot of thought and work into. I've also, however, seen stuff getting rushed because of deadlines or other factors, in which case the blowback was even more frustrating, because we all knew it's correct in some regards, but couldn't just come out and say "yyyeah we agree actually" (no one does that). Sometimes stuff got left as is, because creators stand firmly by their choices, sometimes stuff got corrected 'cause we all knew it wasn't right and were itching to fix it as well, sometimes stuff got changed for sheer fanservice because it became obvious that such change could be profited on (not the best approach, but could be done well depending on the writer responsible).
I don't know what the case is with Larian and Karlach, obviously. Could be an adamant choice they'll stand by, could be a case of some rushed content due to her being added the last, could be something else. I also can't know if they're going to do anything with her further down the line.
But I did love the character, and believe it or not I loved the story, but for me the ability to actually consistently fight for her felt lacking enough that it unfortunately underlined the faults in her questline gameplay-wise, so all I can do now is sit in my chair and talk smack like I know what I'm talking about. (I do try to do it constructively though!)