I don't think the writing is particularly good, and it is particularly problematic in Act 3. The pacing falls apart, all urgency disappears and there is also a big problem with the villains. Gortash and Orin are pretty bad characters and the nebulous blob that is the Nether Brain is not a compelling antagonist. The Emperor is a pretty interesting character, but he sadly doesn't really play out as an antagonist - which I find a massive waste in itself. It also felt like some parts of the plot only make sense if you're playing as Dark Urge.
The companions all being extremely horny and protagonist-sexual gives off a weird vibe, and the progression and design of the relationship system is extremely bad. As an example, Shadowheart can say you're her soulmate that changed her whole life on like the second day you spend together! There is also a severe lack of bonding moments that are purely adventure party/friendship and not avenues for everyone else to hit on you.
There being literally 0 consequences for dabbling in Mind Flayer powers felt weird and bad and generally undercut the impact of the entire main story. There is no reason not to fill out your whole tadpole tree (including becoming half illithid), and there is no reward for completely abstaining. No specific dialogue, no impact on the ending. Not even an achievement.
Lots of small attempts at fanservice for fans of BG 1&2 feel like surface level lipservice made by people who never played the originals. The Flail of Ages being a shitty rare regular flail sold in a random shop is just depressing.
I wish they left more BG 1&2 characters alone if they didn't know exactly what to do with them. Jaheira is mostly fine, but even Minsc felt out of place and shoehorned in and the character assassinations of Viconia and Sarevok just felt terrible. Especially since the role of both of those characters in the plot could have easily been replaced with brand new NPCs.
On a similar note, it strikes me as extremely weird that they seemingly outright refused to have any voice actor reprise their role. Heidi Shannon has disappeared from the face of the earth so Jaheira needed to be recast, but Grey DeLisle (Viconia) and Kevin Michael Richardson (Sarevok) are still out there working for example and Jim Cummings (Minsc) was asking random fans at cons to remind Larian he exists.
I don't think the writing is particularly good, and it is particularly problematic in Act 3. The pacing falls apart, all urgency disappears
Overall I disagree with you. I loved the writing in the game, and the companion back stories are rich, and full of tragedy. But I completely agree with you about act 3. We're smack dab in the middle of literally trying to save the entire world. We just defeated a major contributor to the master plan. We finally travel to Baldur's Gate, close to accomplishing our goal... and we stop all of that to help a little kid find their mommy, investigate dangerous toys, and go all detective mode for a missing prostitute. I couldn't figure out how to get into Baldur's Gate because I had rejected all of those story lines. They felt completely out of character, and not something I had time to worry about with the fate of humanity hanging in the balance. I think that they really could have used a smoother transition from act 2 to act 3.
the companion back stories are rich, and full of tragedy.
The thing is, they're mostly exposition dumped at you. All of them already went through the worst of it and tell you about it. To me, Larian fell in the old TTRPG trap of making up those elaborate grandiose backgrounds for your characters and expecting the other players to be impressed instead of writing the story of their adventures during the actual game.
I was there when Jaheira found Khalid's corpse. I accompanied Nalia when she came back to a ruined home and a dead father. I broke Imoen out of the wizard's asylum.
Karlach told me how bad the hells used to be and we proceeded to make a few trips to the blacksmith together. Wyll told me of his pact and his crazy adventures, the rest just happened to us at camp. Gale told me he banged a goddess and I got to make a persuasion check at the end. Astarion told me of the torture he suffered, and the resolution was done in 2 fights after we met zero vampires before the last room (BG2's Bodhi and her lair were so much ahead of this it's not even funny).
Shadowheart is the only one I felt had a story that I actually experienced with her and wasn't just politely informed of. Oh, and Minthara, but the evil play through really got the short straw in any other way.
That's what a backstory is though, it's what happened in their past, before you met them. If you're experiencing it with them then it's their current journey. I did feel like I was able to reach meaningful and entertaining conclusions to all of the companion's personal stories after hearing their backstory. But I never played any of Larian's other games, so I don't have anything to compare BG3 to. For me it was a completely new style of game, and one I enjoyed so much that I consider it to be a masterpiece, and the best game I have ever played. That's saying a lot considering I've lived through the entire evolution of gaming, starting out with an Atari in the early 80's.