This might be obvious, but to me, it was an important realization that a lot of subreddits that deal with, say, chronic health issues, are going to skew towards people who have extreme conditions. People who get better aren't necessarily going to want to be hanging around in such spaces.

Even subreddits that don't deal with health conditions are going to skew a certain way over time just as a result of who hangs out there all the time and who is accepted, so the "reality" you see in a subreddit is not necessarily the true reality of the greater population. (Again, this should be obvious, but it's easy to forget this when you start reading a reddit post and thinking, "I don't agree with all these people, but there are many people with this opinion. Should I be thinking more like they do?")

I used to think that the people who accused Reddit mods of being "power hungry" were overreacting and were just using that to deflect from their own bad behavior.

Then, weird things started to happen. Some of the things I would post/comment would only get one upvote, and my posts would frequently get immediately removed by an auto-mod. A year or so later, Reddit updated their UI, and the posts with one upvote had a message from Reddit saying they were intentionally hidden.

I conclude it's most likely a combination of subreddits auto-blocking people who post in certain subs, and in some cases the Reddit spam algorithm going completely off the rails. I think a few of my blocked posts criticized Reddit or another social media platform, but most weren't really that notable.

Yes but at least you can choose your bubbles and not have them be chosen for you by an intransparent algorithm.