I think some of the posters have things backward. Hubbard was a well-regarded and much-reprinted writer until he came up with Scientology.
And specifically Scientology, which he started in 1952 after he broke with the Dianetics people.
Science fiction was in one of its cyclical periods of striving for respectability. It was bad enough that John W. Campbell was pushing Dianetics in the pages of Astounding, but Campbell was too powerful a nutcase to buck. After he and Hubbard had their falling out, though, Hubbard was considered as embarrassing to the field as the Shaver mysteries (UFO stories portrayed as true accounts) in Amazing.
As for whether he was a science fiction writer or not, that’s another historic bias problem. In those days the distinction between science fiction writer and fantasy writer mostly did not exist within the field.* Every magazine in the genre, including pure fantasy magazines like Unknown, also edited by Campbell and in which Hubbard was often published, was considered to be science fiction. Hubbard was also well known for his Old Doc Methuselah stories, all of which appeared in Astounding, along with “Final Blackout”, another Astounding piece. Whether he rigorously extrapolated current technology into the future or not, he was considered a core example of a popular and successful science fiction writer.
Hubbard could write. In some ways, at least on the sentence level, he was a better writer than Heinlein and Asimov. His problem was that he didn’t have their vision or story sense, and he was too steeped in the old pulp tradition to make the transition to “modern” science fiction.
His later books, like Battlefield Earth and his ten-volume “drekology” are what really sunk any reputation he once might have had. I can’t say that I finished any of them to give the books a fair shake, but reading any single page was enough to make me fall on the floor in hysterical laughter, so there’s not much chance I will ever get through them.
Even so, that Hubbard and the Hubbard who was a working pulp writer in the 40s and earlier are two separate creatures.
*There were fantasy writers considered as such, but they tended to be British and publish at novel length. The modern distinction doesn’t come about until after Tolkien’s mid-60s success and the launching of the Del Rey imprint at Ballantine in the late 1970s. Even Lovecraft and Howard were considered to be science fiction writers back in the day. The term covered everybody who wrote for the pulps.