"Steady stream" probably overstates it. Keep in mind, again, I'm talking about information I've gathered over a half century. Most people who have these sorts of problems aren't going to talk about them constantly--but they'll still tell stories about events over time, and you also see things like GMs who talk about a top down approach who suggest that their players are all fine with it, and then see some players in a different place that you know are talking about the same game, and have a very different story.
I've also sometimes seen players complain about GMs who are too lenient or favor some players over others. But over time its not hard to start noticing trends in frequency. Add in noting the sensitivity some GMs have to being challenged--at all in some cases--and it doesn't require a big reach to conclude this approach isn't exactly optimal, even if it works for some people and groups.
(And just to make it clear, this can even be a problem with GMs who manage to hang onto players well; there can be all kinds of reasons for that including limited options, player expectations that the next GM will just be the same, or the GM involved being really good at some parts of running a game so that their players will tolerate really annoying top down behavior. Just like you can have GMs who have a very good set of player relations skills who are terrible in other areas).
I come from an information sciences background, so I'm aware of the risks of selective perception and limited data. Yet, at a certain point in life, even with things which there are no experts, you have to come to some conclusions to operate. In these particular cases while I'm aware that I have certain sorts of data sources, I've never seen anyone come up with any sort of explanation of why these sorts of sources would particular bias toward producing the conclusions I've had. I'd at least look at that.
(Its why when someone says "That hasn't been my experience" I'm sincere when I say "I'll take that as a data point". Its just that at this point in my gaming life, I have a rather large number of data points.)