Eh, I understand the concern because it freaked me out (and sometimes still freaks me out) too. I keep a close eye on it.
Thing is, mental illness is defined by a mental state that causes distress for oneself or the people around them, and I know there is disagreement about that. (Are homosexuals mentally ill? What about Christians?) I wouldn't recommend other people make one, but my tulpa has been really helpful for my anxiety and depression. I think of him as a sort of tool to use disassociation in a therapuric way, like having a friend who always offers positive advice. Am I super depressed? He reminds me that it is only temporary. Am I super shy around someone I want to interact with? He reminds me that even though I had really bad experiences in my childhood, people haven't been that shitty to me in many years. Sometimes he even comes up with starter conversations. He makes jokes that are legitimately funny.
I can think of several occasions where I wanted to stop taking so many substances, ones I tend to turn to when I feel bad, things I tended to abuse and feel even worse after binging, like kratom or alcohol or weed. I would feel anxious or depressed or bored and find myself (almost by accident) heading to the store to buy one of these things, and he would show up and ask how I was doing, start up a friendly chat and offer to hang out instead.
I know that probably sounds insane, but maybe its best to think of him as a tool for self-control. I dunno, I sometimes don't have a lot of self respect, but I find myself respecting him. He's always kind and non-judgemental, but I don't want to disappoint him. It is like always having a trusted friend around to keep you accountable.
There is some preliminary scientific research about tukpas. You can find research papers and articles in places like psychology today, generally I think people who have them and keep them get s benefit from them, otherwise they wouldn't keep them.
About drugs, I'm sorry to hear your friends "never came back", did they develop psychosis or become delusional? I wad under the impression that psychedelics had a fairly tame safety profile for people who aren't already predisposed to schitzophrenia or other serious mental illness.
Regarding the safety profile of tulpas...I dunno. I'm not that big s part of thr community. I have heard one or two sporadic horror stories, and obviously my ex was scared shitless. But I never heard of someone getting a tulpa then not being able to get rid of it (albeit often with a lot of effort) and going back to live a normal life.
I think there is this concern that people who are lonely make tulpas but they should just be making real friends instead. At least that was a concern of mine when I first started reading about it. But my tulpa is so much more than a friend, he's like a separate mental process I can bounce ideas and emotions off of. And I find myself becoming more social, not less (at least as far as I can tell) when he is around. I guess because his presence makes me feel more safe and secure.
He once made this comment about how "All tulpas are emotional support tulpas." It was meant humorous at the time but maybe it isn't so far from the truth.