a girlthing

As far as i know hypnosis can work, but its kinda like exploiting a placebo effect.
No matter what, its just suggestions. If someone is in trance and gets ordered to do something they dont want to do they still wont do it.
Its mostly just for removing or at least working on mental blockades. For example when i was a teen i went to hypnotherapy to help me with my social issues and it helped me a bit. Not as much as therapy, but it helped.

Cult indoctrination would be outright manipulation, i guess. The way they typically seem to work is that they give you a sense of belonging and slowly isolate you until the cult is the only thing in your life. Then its easy to control someone because you can either follow the rules or loose all your friends, family and social standing.
My brother works with a bunch of jehovas witnesses, and they arent allowed to even talk to someone who left the cult.

Oh yeah I've straight up hypnotised folks before. I've not had much experience being hypnotised myself though, so I can't speak to what that side is like. (classic top shortage). I wasn't fully convinced myself until one time I hypnotised a sub while they were watching a spiral on my computer screen, and at a few points I briefly walked between them and the screen while they were watching the spiral, and then when I brought them up they didn't believe me that I'd ever blocked the view of the spiral.

I've heard of cultish or abusive groups within hypnosis circles, (IIRC something called CORE or CORPS or something, notably distinct from KORPS which I've only heard good things about) but I don't remember any of the claims clearly. My understanding is in line with the commenters above, though - that hypnosis on its own won't brainwash someone or make them go against their own desires - it's a way of occupying someone's conscious mind (and lowering the associated conscious barriers), but not removing their will entirely. It seems plausible that if someone was manipulated in other ways, though, hypnosis could be used to reinforce that abuse.

Honestly, hypnosis is kind of a "fake it till you make it" situation, at least for me. I started out not being able to feel much, but reinforcing triggers with my partner has gotten them to a point where I do feel that controlled, "automatic response" kinda thing. sleepingirl is a really respected creator in the hypnokink scene, she's got a site breaking down the basics and how it can work in a kink context: https://www.learnhypnokink.com

Big line of hypnodrones in storage pods all collectively thinking, "Haha wow, these people really think they can control me with weird lights and sounds, I'm just following the orders because this stuff makes me happy and I get free housing and healthcare."

as a more experienced hypnokinkster, some of hypnosis is quite literally playing along and hoping it works. that's what i recommend to a lot of people who want to do hypnotic amnesia for example. i've been on both sides of the pocket watch so to speak and it does work.

when it comes to the ideas of "mind control" and "brainwashing," those also do work outside of cult indoctrination. however, that's all on a spectrum and "brainwashing" is a specific set of behavior modifications made versus just hypnotizing people and doing a scene. i do know people who have been brainwashed, i've brainwashed someone else in some ways (with proper consent and negotiations beforehand as well as limits placed) and it just depends on how much you're into it, your partner is into it, and how much you know.

a lot of hypnotic experience is based on framing and priming. before hypnosis was the whole "you are feeling sleepy, you're going to flop, blah blah blah" thing, it used to be people acting almost as if they were having an exorcism done to them. people would often convulse and it was a much more active experience. so what changed? well when a farmer was hypnotized and reported feeling "sleepy and relaxed" our framing around hypnosis changed and what people were primed to think of about hypnosis was also changed. (this is a tldr of hypnosis history, there are many people who know a lot more than i do and can point you in better directions of active resources)

you've been primed to think of hypnosis as roleplay so you're much more likely to view it as roleplay versus someone such as myself who has experienced it, and so i'm primed to experience it a certain way. i have a friend who runs a whole class on this.