The companions are typically quite annoying. People make the comparison to Critical Role and I think that's a good starting point of reference. Everyone in your party is super-special and most of them have utterly ridiculous special backstories.
That's not writing, that's character design. A lot of people here confuse bad writing, bad story, bad character design and so on. The writing here is mostly solid with some occasional bad stuff. The character design is what some have problems with and later on with the story.
The backstories, and the way they're presented, can surely be classed as "writing". I'm talking stuff like Gandalf popping up to loredump Gale's crap backstory, Karlach just overwhelming you with her story of how she became Major Colonel Sargent General of the entire universe as soon as she meets you, stuff like that.
Don't know about Gandalf, but the example with Karlach is a good one, that was indeed bad writing. But for the most part, its actually pretty good, froggie, shadowheart, the vampire, various conversations with Gale.
Frog, Shadowheart and Astarion have the most complete experience regarding storytelling and content. Given that you are playing the game in their quests at creche, shar temple and cazador hideout. Those are the three characters that I felt something.
Now Karlach just feels rushed, she got that strong cheerful personality but the content around her is so raw. Dammon you expect him to come out with something or just say shit is fucked but at act 3 he just stay there saying yeh yeh gonna figure out something, so feels like they didn't have time to finish her content. Also Gortash dialogue with her was awkward as shit besides his pathetic fight that I took a huge dump on his face.
Gale feels buggy and slightly schizo they wanted to make him lusting for Karsus power but he never managed to express that darker side and just have a superficial exposition simping for Mystra. I mean you can convince him to go for Karsus powers but it feels so artificial with all the standard dialogues the character expose.
Minsc I have no fucking idea what he is doing in game besides being a prize for Jaheira quest?
Jaheira its all right I guess besides the weird power loss, nostalgia fan service but has content to back it up and given previous games you don't need much to acknowledge this character existence
Halsin lose any purpose after act 2, he just tag along because fucking reasons?
Wyll I gave some thought at first I had a bad impression but someone here in this thread said that he is a noble character but a dumb fuck at the same time and thats true that made me sympathize more with this character felt believable many instances of idiots thinking they are doing the right thing by selling themselves and get fucked in the end. At least the game lets you make him see this failure.
Minthara as people said seems like its bugged as fuck and you can't even enjoy whatever the fuck content it has.