From the top of my head, i would say that in terms of computer action rpg's, both Bloodlines and New Vegas have better dialogue and story, with the added bonus of non linear gameplay and level design.
My problem with this game's writing is down to the developers trying too hard to emulate the source material, they use violence, foul language and the "ugliness" of the "bad" characters and the big bad unjust "world", there is no respite but for the constant fighting, murder, betrayal and rape... just because in their minds they think that's what the books are all about (they aren't).
Considering the source material, yes, i think it's a badly written game, that does an injustice to the world and characters Martin crafted.
To each its own I guess. Bloodlines was released in 2004 and it is debatable if it is "modern". New Vegas is a good game. Probably the best in the last 7-8 years. The plot however, is so so (on the better side). I mean, I really'd like to separate quality of dialogs from the plot itself. Game of Thrones has by far the better plot than NV, though dialogues may have been worse. VO is another issue. It is not consistent in the case of GoT. Some are very good, while some are bad. But I wouldn't call NV the pinnacle of voice acting either. It was serviceable, and that's all. Loads of bad acting in it as well. Bloodlines is a masterpiece, but then again, how many games of that quality VA you've had ever?
I don't agree with you when it comes to "bad adaptation of source material". You cannot contain in 15h game the complexity of 5000 page book. It would be more fitting to compare GoT game, to merely a few chapters or a sidekick story within the book. And it did fine. I don't know, where you got the idea that everything was collated to "uglines", "bad", "foul language" and so on. Yes, it is unjust world, like in the books. Yes, world is portrayed as unforgivable in which you need to pay your deeds sooner or later, and there will be no fae coming and kissing your ass and absolve your sins. But there is also an element of "good" in the whole story. Mors is fucking example of lawful-chaotic good character depending which ending you choose. And it is not only about the ending. Throughout the story he is a good chivalrous person. And there are some characters which are good as well. The thing is, that the characters that you perceive as "evil" are often forced to do so in order to survive. There are exceptions of course, like main villain or Cersei, which seem to be completely evil, but again, it is not a whole saga, but merely a snapshot of their take on the events portrayed in the story of the game.
Alistair, for example is a very nice example of the character turmoiled with his past deeds. He did wrongs and try to right them, but as you learn about why he did it, you realise that his choice was between "worse" and "worst". Either he will betray his friend, or he will lose everything (lands, family etc.). Don't fucking tell me that it is not an important part of the GoT idea (e.g., Jon Connington case - during rebellion forced to go exile loosing all the lands because he decided to stay with the king and yet proclaimed the traitor by the same king).
Overall, during the course of the game he is not a bad person though. He merely tries to find the best solution out of the shit he is sitting in. In the end we learn that he was a doomed person no matter what he will do. Either by people around him or by his own conscience. In my book, he is a great character (probably one of the best "tragic-hero" characters in games ever).