I have a suspicion that all Whedon shows age terribly. You can definitely grade them on a curve, based on when they were made, the fact there was little to no prestige TV at the time (and so everything that requires tons of wacky sets and special effects looks like dinner theater), and that a lot of writer rooms sort of lifted his best qualities and distributed them across subsequent TV and movies, retroactively making him seem less innovative...but whatever the reasons, fair or unfair, I think the end-result is just not very rewatchable.
That's my experience, at least, and I really really liked Dollhouse and Firefly at the time. I also rewatched the first Avengers recently and thought it really held up, with a waaaaaaay better sense of story, pacing and characters than Endgame or Infinity. And Cabin in the Woods is great! Still!! The guy could write.
But something seems to have happened to him after that first Avengers movie. Age of Ultron is just limping through story beats. Justice League? The Snyder Cut's no masterpiece but it revealed that the worst, just laughably bad parts of the original were almost all Whedon. And then whatever happened with the Nevers, who knows. But it's like he just imploded, creatively, around 2012. It's obvious now that he was always a complete naughty word to work with, but that's not the only thing killing his career. He was in a tail spin well before Ray Fisher exposed him.