Everyone probably has experienced dreams where they stumble upon a powerful, deep, great idea. Upon waking up, the idea generally turns out to be trivial.
Psychedelics and entheogenics often give you the feeling of profundity. They put you in the same mind state you have when you contemplate a truly deep thought, but that doesn't necessarily mean the thoughts are particularly deep.
That said, that doesn't make the experience entirely valueless. First of all, it's fun and interesting, just like riding a roller coaster, reading a book about optical illusions, or listening to music.
Second, many of the insights gained are actually valuable. Not as much as they initially appear under the influence of the drug, but still valuable afterwards. It's an occasion to explore thoughts under many angle, to make connections between unrelated topics. You'll end up throwing out more than half of the ideas the next day, but a few might stick.
If you think about making a breakthrough, it is a two stage process. First you have to think the thing, then you have to assess that thing and come to the conclusion that it is a breakthrough. And that in turn makes you feel a certain way.
Often when we dream, or a hallucinate a 'breakthrough' - we're not actually dreaming or hallucinating the thing itself, we're dreamng or hallucinating either of the following stages, we're imagining the experience of assessing something as a breakthrough, or just directly feeling the emotion that would result from a breakthrough.
Because of the way our brains are wired to see causality, we assume that it must be because we actually had the thing that caused the breakthrough. But, tantalisingly, we can't remember exactly what it was. It was awesome, perfect, we remember that, but what exactly, that's gone now, damnit.
I think LSD can definitely expand the mind in the sense of letting you experience the mind in a different way. But I'm very skeptical that 'insights' can be brought back from a trip. The excitement and emotion of that insight, certainly, but the actual insight itself seems never to be quite explicit.
> “If the doors of perception were cleansed every thing would appear to man as it is, Infinite. For man has closed himself up, till he sees all things thro' narrow chinks of his cavern.”
Also, selective amnesia is a common effect of various drugs, including high-dose LSD trips. Dreams are also easy to forget upon waking, leading some to recommend that you try to make what you do remember from dreams (and from trips) concrete in ordinary waking consciousness, through artifacts such as art, music, etc.
Some psychological theories posit that there is a mechanism of repression that occurs when the mind resists the brining of certain subconscious material to consciousness. This is one reason that some therapists recommend one does not venture too deeply in to the subconscious without a trained guide. You might not be ready to handle what you find on your own.
Another issue is that it can be very difficult to stay focused long enough to transcribe what you're experiencing mentally in any sort of comprehensible way. Just as you're a few words into your description of a vast and intricate concept, 5 completely new ones come rolling in.
Not that I'm any sort of expert, but the solution seems to be various forms of mental discipline. Meditation and trying to remain a dispassionate observer (versus an impassioned epiphany seeker) can help to slow things down and make the experience less chaotic.