Are you kidding? have you seen the drunks, murderers, thieves, adulterers turned into saints immediately through the Holy Spirit. over a million miracles happen each year, many medically verified (see Dr. Craig Keener's huge book on miracles). all of this is very well documented so that it is extremely probable to say that these things happen by chance or other than God
I'm aware that hallucinogens can cause "mystical" experiences that'll transform a psychologically ill person, sometimes in a single afternoon. There's been a resurgence of scientific research on this in the past decade or a little more, with astounding results. A few doses can potentially cure a treatment-resistant depression, a longstanding alcohol addiction, or the terror of death among terminally ill patients.
Is that the sort of thing you're talking about, except about altered states that are religiously induced? If so, then no big argument from me about that end of it.
IMV it's not the claim of a fast and radical transformation that's hard to credit. Rather, it's that you take people's testimony about WHY it happens too unquestioningly. People clothe their interior experiences in an immense variety of stories, which is very wonderful. But I don't know that they're right to project that symbology outward from headspace onto the outer world.
I'm an agnostic atheist with an interest in religions precisely for this reason. It's potentially transformative of the sense of self in truly astounding ways (for good and bad). IMV that's a big part of what religion's been about for many millennia now - a transformed sense of self.
The trouble is they'll "metaphysize" it -- by which I mean, they don't leave it as a phenomenology of their inner world, instead they think it's descriptive of an esoteric structure to the cosmos. And that's why their testimony about "spirits" from "the beyond" that are external to themselves isn't trustworthy. They're using mythology to explain the workings of nature, when mythology is, at best, lessons about inner transformation.