Only if you assume that the episode was about Harry and the peculiarities in the dramatic structure were merely incompetence can you really argue that Non Sequitur was a Harry Kim story. Otherwise you're begging the question of what the story is about.
As you point out, the episode hands Harry Kim exactly what the character wants (to be on Earth with his fiance), and the character doesn't struggle for a single second whether or not to leave. If that's not incompetence, then in the very least, it's not very dramatic.
I have no idea what it means to say Data sacrificed himself, since there was no plausible reason to think he couldn't make up a backup disk. As to what Picard, I have no idea, but that seems to me to be pretty good reason to regard that script as a failure.
Data's technology was demonstrated as being impossible to replicate on the series. Is that plausible, given the level of computer technology displayed in the rest of Star Trek: The Next Generation
? Not at all, but if you're looking for scientific plausibility, you won't find it anywhere near Star Trek
Of course, B-4 was designed as a trapdoor in case Spiner wanted to return to the series, but the lack of a sequel rendered that moot.
As for reasons to regard that script as a failure, coming up with a comprehensive list would require me to re-watch the movie, something I'm not particularly inclined to do at the moment.