Talking about the penultimate episode alone, it seems to be review bombed due to the following:
- Homophobes pissed about the coming out scene for Will (apparently the one star reviews are heavily coming from Saudi Arabia).
- Crazy "Byler" shippers who are upset the romantic pairing they were pining for isn't going to happen.
- People who are genuinely underwhelmed by the episode.
I think the latter is at least understandable. The penultimate episode didn't have much happen. There were no new realizations about the Upside Down or Vecna. What they did with Holly was a cheap-ass move, with Vecna just waving a magic wand more or less, which (at the moment) seems to invalidate her entire escape arc. There were no big action scenes with the demos or Vecna. There were a fair amount of the leaden exposition and planning scenes which have made this season so lackluster.
The Max/Lucas reunion scene was great, but it belonged at the end of the previous episode, not the beginning of this one. Seeing Steve/Dustin vibe again was great, though they put their issues to bed in the previous episode. Whatever they're trying to do with Kali is so underbaked I don't think it's hitting - in part because Eleven spent a single day with her, and it's not like they built a tremendous rapport at the time. I think they fucked up by leaving the Kali reveal to Episode 4, because they haven't had time to reestablish her.
I can see being let down by this episode, but not considering it the worst ever. Chalk one up to the homophobes, I guess.