Nirvikalpa Samadhi is not Advaitic Gnana.+

Sage Goudpada: ~ To establish the truth of Nonduality by sheer reasoning alone. He begins by defining "What is real?" "What is unreal?" etc, because that is the right way to discuss or teach. People must first know what they are talking about. (Manduka Karika)

The reasoning is interpretable in two ways. The egocentric interpretation is to apply it only to the waking experience. The Soulcentric interpretation is to apply it to the three states. The latter leads to a final settlement of the problems because it considers all data.

As we keep digging deeper and deeper, the truth will shine as pure awareness in the midst of the duality (waking).

The Soul, the Self is our ultimate teacher. It is only our inner teacher that will walk with us to the goal, for the Soul is the goal. From the ultimate standpoint, there is no teacher, there is no teaching and there is no student.  Millions are searching the form truth, but one in a  million will realize it.

Sage Sankara definitely says that yoga is not the means of liberation (page 132-133 of his commentary on Brihadaranyakopanishad). 

Bhagavan Buddha gave up his austerities of yoga as impossible and useless. (Page.70/71 "Buddhism in Translation” by Warren)

Bhagavan Buddha got enlightenment only after he gave up Yoga. Unless one exercises his reason--there is no chance of getting the truth.

When the yogi enters this highest Nirvikalpa (effortless) Samadhi, he will at once enter deep sleep. This will make plain to him after he wakes, that the inner self he sought and found, the Atman, is reached only when all his ideas are refunded into it, when there is then all the features of nonduality, one without a second. However,  the yogi must later wake up, emerge from Samadhi and there is duality again, for the world of objects confronts him. So now he has to work on the next stage which is to find consciously in the waking experience the same non-duality that he unconsciously knew in sleep. This is done by learning that the universe is or idea or an object for the formless subject, and then refunding the universe,  ~idea back into its source, which is the Soul, which is present in the form of the consciousness. Only at this final stage, he says "Atman, which is present in the form of the consciousness is the same as the Brahman. The Brahman is the ultimate truth.  " Now he is fully aware of it.

All yogic visions, however, wonderful will pass away; they go as they come. They have the value of dreams. They are not truth which is un-passing and beyond changes.

One can’t shut his eye to the universe, which confronts him as in Samadhi of yoga and see supreme reality. One can know it only by keeping himself clear and open.  Sage  Sankara says:-   The yogi must add discrimination to his quest.

Nirvikalpa Samadhi and deep sleep are the same from the non-dualistic perspective,  the absence of the known. The knower was there. How does Samadhi give Gnana? Only by preparing the mind to see that the world disappears and reappears and, that nonduality is here and duality there, to convince the man that in non-duality one won't disappear as he does disappear in Samadhi or deep sleep. Another advantage of Samadhi is one gets the capacity to forget the external world and to treat it as an idea.

Yoga can never give you the fundamental thing, that the world is an illusion. Only Gnana can give it. Nirvikalpa Samadhi is unquestionably the same as deep sleep, and all ideas are refunded back there too. One must learn what ideas are when all the ideas of the universe's existence go back into one’s mind through Yoga. Then one learns this. How has he to learn that the entire universe is consciousness or Brahman if he stops at Nirvikalpa Samadhi? Without perceiving the universe, and having a duality before him, it is impossible.

Nirvikalpa has no duality, hence it cannot tell you about the universe. The yogi who emerges from Samadhi and says he found Gnana there, says it to a second person, hence there is duality again. If he were a real Gnani, there would be nobody for him to tell that he had experienced Gnana.

Self-knowledge or Brahma Gnana or Atma Gnana will interest only a few people; the rest are interested in Religion, yoga other paths, and pleasure hunting. :~Santthosh Kumaar