From all that I've seen of the game I also would agree that Jaheira is a great character whereas Halsin I could do without. But I, as someone who does not believe Larian's writers are particularly good, would argue the reason Jaheira is a great character is precisly because she's a character origially developed by other writers in another game, and not a truly original Larian character. Jaheira is awesome in BG1 and 2 also, and has always been in every one of my great many playthroughs of those games, and is also always my only romance interest in those games.
The other companions are very well written too, with the exception of Halsin - some of their stories had me moved to tears actually. I would argue, that it is even harder to take an established character and make something new with them without changing their personality - and that, they did well with Jaheira - and Minsc for that matter. I would even go so far, that I like BG3 Minsc more than BG 1&2 MInsc.
"We are all stories in the end. Just make it a good one."
