Yoga helps to train the restless ego to get steadiness, but never Truth because it ignores the external world.+

Yoga can yield the only duality, because everything that one can do or practice becomes a vanishing 'known.' It yields relative truth based on imagination, which is true from the physical viewpoint of view, not the non-dualistic truth, which is the ultimate reality.

In Gita Chap.IV where Lord Krishna says: ~ "This yoga has been lost for ages" the word yoga refers to Gnana yoga, not other yogas: the force of the word this is to point this out.

Lord Krishna describes some of the other yogas but devotes this chapter separately to Gnana Yoga. So one sees even in those ancient days people did not care for Advaita; they wanted religion; hence Gnana got lost. That is why Krishna calls it "the supreme secret." Krishna points out that the yoga must-see "Brahman in action."

Sage Sri Sankara in the commentary to "Brahma Sutras:~ " "The highest beatitude is not to be attained by Yoga." (Sacred Books of East Series page 298 Vol.1.)   And he also says Samadhi is the same as sleep (p.312) ---this indicates that yoga is not the means to Self-realization.  And yogic Samadhi is not Advaitic Self-awareness.

Panchadasi: - the impossibility of yoga arrives at a successful end to its practices. (P.509 v, 109)

Yoga cannot remove ignorance. It is only a step. It removes obstructions. Yoga will help to train the restless ego to get steadiness, but never Truth because it ignores the external world.

Our chief argument against yoga is that it shuts its eyes against the world and then has the temerity to declare that it knows the world to be Brahman! Because it has not inquired into it, it knows nothing.

Yoga's secret from the Vedantic viewpoint is this: it helps the yogi by giving him the feeling that the world is not worth bothering about, it detaches him from the world; it makes him treat the world as a dream, i.e. an idea. It does the same to his ego to some extent because he becomes indifferent to what happens to him. But the great secret is that this is only feeling, he feels these things only but does not know that the world is an idea. Such knowledge can come only after philosophic inquiry and in no other way. That is why yogi cannot be Gnani. It is the difference between feeling and knowledge. 

Feeling of the yogi that the world is unreal may change tomorrow because all emotions are liable to change; and the fact is that yogis do change, as when going after women they lose their sense of world unreality though previously they felt it.

A permanent view of the world as unreal can come only after intellectual inquiry; such knowledge cannot change. Were the yogi of sufficiently sharp intellect he could discover the ideality of the world by reasoning alone and then it would not be necessary for him to have gone through yoga practice at all; that is why we say yoga is for dull or middling intellects.

Bhagavad Gita Chap.IV: "He who achieves perfection in Yoga finds the Self in time." This means that after his yoga is finished, he begins the inquiry into ultimate truth and in due course this inquiry produces the realization of the universal spirit as the result.

All other yogas lead finally to Gnana which transcends and fulfills them. The highest form of yoga is Gnana Yoga, according to which the individual soul realizes through knowledge its identity with the universal Soul or the Self.

It is not possible by mental control alone, by yoga, to achieve Brahman, but at best one falls into asleep. It is like eating fire or leading an elephant by a thread or draining an ocean drop by drop, to try the yogic way. When the yogi shuts his eyes and does not see the world he is like the cat in the Indian proverb who shuts its eyes when drinking forbidden milk although other people are there, and it imagines it is unobserved because it cannot see them. He does not examine the phenomenal world and hence cannot see Brahman for he takes that world as real but runs away from it.:~ Santthosh Kumaar