The problem I generally see is too much confidence in mystical knowledge.

Rationalism vs. Empiricism vs. Mysticism  

While I think that there is a kind of knowledge that is not from reason and not from empirical observation, still that does not seem to be carte blanche  permission to assume all mystic experiences are a valid source of information. And even if it was, it would only apply to ones own experience. You would not be able to depend on someone else mystical experiences as a source of information for yourself on how to live your life.
There is a kind of sneaky attempt to get people to believe in the mystic experiences of other people by calling it אמונת חכמים faith in the wise. They pick some charismatic insane teacher with mystic delusions and decide to call him wise.

We don't have prophecy anymore. And in any case prophecy seems to be a source of information separate of mystical experience.   Furthermore there is such a thing as סוף הוראה the end of the period when it is possible to make a halachic decision.
This may not seem like a big deal to most people. But from where I come from what I see a lot is people that are depending of the mystic experiences of some loving, lunatic leader deny the other areas of information. To them the only source of valid knowledge is the mystical experiences of their beloved leader.
Though they will use pragmatic reasoning in their daily lives but as for any major decision they will go only with the mystical experiences of their leader.

Why this came up is that I don't think knowledge of morality comes from mystic experience. There are some principles of morality that I think we know by reason. דרך ארץ קדמה לתורה. Others we need Torah to reveal to us. And to understand Torah we depend on the Sages of the Mishna.  We no longer have prophecy and mystic experiences can't cancel the Law of Moses, nor alter its meaning.
Sadly by means of Kabalah learning that is exactly what people do do. Yet people are looking for spiritual wholeness. Where can you find that except by mystical experience?
Though there are substitutes, they do not seem  numinous unless you endow them with numinous meaning and content.

I any case I had a few issues to bring up about this. One is the Rambam. לא הצם והמתפלל הוא הנרצה אלא היודעו. "Not he who fasts and prays is acceptable to God, but rather one who knows Him."   The kind of knowledge of God that the Rambam is talking about is explained in other places in the Guide. It is a kind of knowledge that comes by learning Torah Physics and Metaphysics. He is not talking about mystic knowledge.

The problem I generally see is too much confidence in mystical knowledge. This leads members of cults to all kinds of terrible sins.
  It is not that there is no mystical knowledge. Just that there seems to be too much confidence in other people's mystical knowledge  and that one is supposed to be convinced that they are "tzadikim" and that is supposed to overrule the basic common sense  explanation of the Torah  and common sense in other areas.

 The idea of authority is something we all use. We believe our Physics textbooks without doing every experiment and calculation ourselves. But mystic knowledge can't override the basic explanation of the Written and Oral Law.  When people think their leader has mystical knowledge that can override common sense morality that is when they get into problems.
And I think this was the problem the Gra was addressing when he signed the excommunication. I think he thought that over confidence in mystic knowledge was held to override the Oral and Written Law and he wanted to put a stop to this phenomenon (with zero success as far as I can tell).

I should write a whole new essay on this because I have not  even gotten to the issues that are bothering me which are the Rambam's idea of knowledge of God. What can this means?
Does it mean the Infinite Light? That seems unlikely. After all the Sefer Yetzira itself calls it אור נברא Created Light. This is because the Sefer Yetzira is trying as it should to preserve Divine simplicity.
Or is it Devekut? Or mystical experience of God's light? I clearly need to deal with this at some future date.