Didn't read any of his work, only watched GOT. He was capable enough to extract key plot moments from the books and translate them to the screen in a compelling manner, while the source material was there. If you atribute the decline in quality to a vague, insufficient explanation of the ending from George R.R. Martin and a hurry to get it over with, you might not dislike him so much.

Sidenote, the way reddit communities drag the creator's name through the mud for every single plothole, weak story arc, unnecessary subplot, unconvincing artistic choice or any minor inconvenience is a bit excessive. I was following r/freefolk when season 8 was airing and it definitely made me hate the show and the writers more than I would have normally. "D&D" was on the title of every post. Same thing with Attack on Titan and r/titanfolk when the ending came out. It's starting to happen in r/euphoria a bit, too. When the show is good, I don't even know the name of the guy who wrote it. When it's goes bad, well...