The writing is on the level of the first two Baldur's Gate games, possibly a bit better as it isn't completely drenched in ideas that were already hackneyed the first time Tolkien picked up a pen.
That doesn't make it good, but it doesn't need to be.
The first two Baldur's Gate games may have a (more cohesive) Tolkien-y vibe rather than a Marvel vibe (which IMO BG3 has in spades), but if you look past vibes and look at the actual storytelling, the originals are far superior in terms of narrative storytelling.
For example, a very basic thing: act structure. BG1 and BG2 both follow a very solid act structure that is tried and true for a reason. 1. Introduce main protagonist, introduce threatening main antagonist. 2. Protagonist gains power. 3. Protagonist suffers a personal setback at the hands of the villain. 4. Protagonist claws their way back to being able to threaten villain. 5. Protagonist defeats villain. This applies to BG1 and BG2 both, even if the details differ.
In BG3, the act structure is a mess. 1. We have the protagonist (who can have no narrative connection to the story at all) and one potential villain (maybe ally!!! Or maybe it isn't even the Emperor who tadpoles us, who knows!!!) introduced. 2. We have some more potential villains who could also be allies introduced (Raphael). 3. Protagonist gains power and ends up in the shadowlands. The threat introduced at the beginning is neutralized and the focus of the story totally shifts to more of Larian's own characters who are much more interesting than Tav.
(I enjoyed Act 2 the first time I played it, but it does not fit with the rest of the game at all. It feels like a drawn out sidequest where SH/Aylin/Isobel are suddenly the main characters.)
4. More villains are introduced. Protagonist suffers absolutely no setback. 5. Protagonist goes and defeats whichever villains aren't allies. The end.
Durge at least gives the protagonist more narrative relevance other than being the player's self-insert, but the plotholes surrounding Durge are terrible. Durge is also, IMO, a pale shadow of Charname from BG1/BG2, a character who had actual narrative weight, development, and agency through two games + expansions rather than being one seventh of a protagonist.
So yeah, tl;dr, if we are talking narrative storytelling, BG3 is not nearly on the same level as the originals. BG3 has great companions; it has great player engagement elements that have nothing to do with the story. It's more of a sandbox game than a story game, which is great for a lot of people, but not to my personal taste.
