The problem is that they find it incomprehensible that anyone likes anything made by Joss Whedon at all. One of the complaints is about the characters. There are too many and they are all bland, boring and poorly written.
I kind of doubt anyone who says that has actually watched
Buffy or
Firefly or
The Avengers, but okay. Still, nobody bats 1.000 - I couldn't get next to
Dollhouse - and sometimes a writer's style just doesn't connect. I'm apparently immune to the charms of Harry Potter, for instance. I read the first book and saw the first movie, and couldn't tell you a thing about them.
One of the things they've said is that even at the time, shows like Buffy and Firefly were considered terrible and all treated poorly by reviewers.
Well, the 15 reviews collated by Metacritic from 1997 give an average score of 80, which is outstanding. The links to the actual reviews are all dead, though, and Metacritic's methodology is to assign a 0-100 numerical score to every review, even if the writer themselves didn't (and almost none do). So Metacritic scores are actually Metacritic's evaluation of the review, not the reviewer's evaluation of the material. Still, an 80 is miles from "treated poorly."
Firefly is a slightly different story. Its Metacritic score is only 63 on 30 reviews, but that's misleading in a couple of ways. First, the reviews are all over the yard. Many critics gave it outstanding reviews and many gave it terrible reviews. Perhaps some of them shouldn't have written a review of the show in the first place, as they were unprepared to like it. The NY Times reviewer wrote, "Firefly is even more of a confusing mess than the description makes it sound. It's a crazy quilt of "Star Wars," "Mad Max" and "Stagecoach," just to mention the most obvious films it calls to mind." She clearly didn't care for what it was attempting to do, so whether it succeeded or not perhaps becomes irrelevant. I suppose someone who doesn't want to see even a good comedy-adventure, sci-fi western might benefit from a review written by someone who also doesn't like those, but in that case, I would think the trailer would be enough to tell you it isn't your cup of tea.
The other issue
Firefly faced was the infamous mishandling by its network. The first episode aired, for example, was actually the 2nd or 3rd episode. No joke, Fox aired the series out of order (bring this up with any "Brown Coat" even today and you might see Dr. Banner turn into the Hulk - or Jennifer Walters turn into She-Hulk, as the case may be

).