Is it dangerous? Two words, Shabtai Tzvi. The modern version is based largely on one direction, that of Yitzchak Luria, the Ari, and his school. While he promoted a resurgence of Kabalah and the mystical side of Judaism, he also introduced new ideas that cannot be traced back beyond a century earlier. The rejoinder to this is that Kabalah existed in a hidden form, where only a few people passed down in a clandestine chain. To me, this breaks the whole Jewish model of generational transmission. The idea was that everyone saw at Mt. Sinai and that other religions were passed down by a single person, as with Christianity or Islam. This even spawned the circular Kuzari "proof", which we have already discussed. To have a major change in the religion that only a select few know is not Jewish, because in Judaism, even the king, prophets and saints are not above the law. However, if only an oligarchy know the "true" law, then no one can ever hold them accountable. Even Moses took pains to be seen by the people speaking with G-d, before he would be trusted to transmit the law.
To answer, vivid dreams are the same as any other dream. They reflect what we think about during the day, as he says in the Raisha of the article. Our minds simply prepare us to do something that our bodies are not yet ready to do. When our brains tell us something, we tend to believe it, just as we tend to believe our parents when we are children. If our brain sends us false signals, which we are used to interpreting one way, we are impressed that we have experienced "reality", even if it is weird. Stage magicians understand this. They tell you things and show you things normally interpreted one way, and then they change the field so that you are tricked about what you have reasoned will or can happen next. Usually, when shown the secret behind the trick, people are amazed at how easy it is. Stuxnet virus also works on the same idea. It tells the instruments in the control center false things that are normally interpreted as true by the trusted source.
Overly trusting our brains, any more than our eyes and ears, leads us to endangering ourselves based on hallucinations, or little voices. A vivid dream is no more than a vivid hallucination. If you put your old aunt in the home for believing it, it is not valid for you to believe either. Mental wards are full of people who follow the reality of their mind, when in everyone else's reality, their mind is what is broken.
Just because we receive a message from our minds, rather than from our outer senses, it may not be any more trustworthy. This brings a certain amount of discomfort and insecurity as we deal with the world around us, but it is a truth that we cannot deny. And this is sufficient for he that understands. The rest is passed down only to special students and only the chapter headings.