In the past, memes and technologies were largely contained to specific societies, so it was possible for a bad meme to cause the downfall of a civilization, but not all of humanity. Now, these experiments happen at such a pace and spread over such a large fraction of humanity, that a crash could affect nearly all of us, such that recovery might not be possible. So, even if the risks are low that any particular 'experiment' has bad effects, the possible consequences are huge.