Having partial insight into the nature of reality, without the non-identification that comes with awakening, can make one's suffering a lot more apparent and "real" feeling.Fair, but this is a problem with any kind of learning. "A little knowledge is a dangerous thing." It applies to going off to college as well.
Personally, I believe that issues I mention above apply to psychedelic use as well, i.e., it's possible to stumble into early stages of insight via drug use. This can be even trickier than when it happens via meditation, because typically drug users are not equipped with a framework to make sense of what's happening.
With drugs, you can't get out of the experience easily. High-power dissociatives can lead to a state where the person forgets being on a drug entirely, and time dilation can make a few hours of hell feel like months or years. That's the dangerous part. Drug use in an unwise setting or with the wrong people can lead to a panicky thought-loop that characterizes most bad trips. In the worst cases, it causes HPPD or even PTSD, both of which can be long-term afflictions.
If you're meditating and it becomes uncomfortable, you can stop, open your eyes, and come back to a normal state within a couple of minutes. Meditation can lead to difficult emotions (as can reading a book, taking a walk, etc.) but one isn't stuck in the experience for hours. Meditation, over time, gives you control over your state of consciousness; psychedelics hand that control over to a chemical.
Does meditation on its own ever lead to things like psychosis? Well, I've practiced at a number of meditation centers in the U.S. and Asia, and I've certainly heard plenty of stories of what happens when psychologically unstable people come to do an intense period of practice...
Disclaimer: I'm not a psychiatrist.
A few things. First, a lot of psychologically unstable people use "Eastern" approaches like meditation and yoga in lieu of proven psychiatric treatments. Bad idea. That's quite dangerous. Accomplished zen masters may have no need whatsoever to pursue mainstream psychiatric help, but that doesn't apply to the rest of us. The idea that one can overcome biological mental problems through "spiritual" means alone is dangerous, wrong, and no meditation teacher worth his salt would insinuate such a thing.
Most Buddhist adepts that I've known have said they would never advise someone to stop taking psychiatric medications, any more than they'd advise someone to stop taking heart medicine.
There's a misconception about Buddhist attitudes toward "suffering", the first noble truth being quoted as "All life is suffering." Actually, it's closer to, "There is suffering." The word for suffering, dukkha, just means painful emotion. Painful emotions are a part of human existence because we grow old and die, because we long for permanence in a dynamic world, and because we're biological organisms prone to Hamlet's "thousand natural shocks". That doesn't mean that clinical depression should go untreated. Not in the least. It means that even psychologically healthy people are prone to unpleasant emotions as long as they are in cyclic existence (which is not depressing or "dark", just obvious.)
Second, dramatic lifestyle changes are always risky for unstable people or even stable ones. That could be moving to another country, having a child, changing career, or (yes) starting a spiritual practice. It's best to ease into such things gradually if possible.
Third, going from no meditation at all to 2-week retreats is not a good idea. It's not a sign of spiritual progress. To me, it looks like a sign of mania. Again, the danger there is not from the spiritual practice, but from the frenetic, manic state of mind that produced the sudden change in spiritual orientation.