It can be a wonderful resource if done right, as you noted. The problem is, members of certain groups often
don't actually help each other so much as they egg each other on. Gang-stalking support groups are a good example. Sure, it's nice for people experiencing the same delusion to have a place to discuss it, but they end up simply validating each others' delusions, for the most part. No one gets better. If anyone does get better, they tend to leave. (To return to the example of gang-stalking groups, members are likely to be chased out if they begin to verbally doubt the narrative.)
People compare this kind of thing to group therapy, but actual group therapy tends to have a leader of some sort, often a professional, who can help guide the discussion in a healthy way. Without that, all I see is people wallowing in misery together and becoming more extreme. This goes for incel groups, "femcel" groups, pro-ED groups, conspiracy theorists - anything you like. Most mental health support subs on Reddit are dumpster fires. They're great if you want to complain about, romanticize, or meme your experiences, but the majority really aren't useful for
healing. I don't think there's a solution to this problem. It is simply a phenomenon that tends to happen when people form interest groups and become devoted to them. In some types of groups, it's not even a bad thing. In other types, however...
Well, you end up with bomb-makers.