The Bhagavad Gita with Commentaries of Ramanuja, Madhva, Shankara and Others.
Bhagavad Gita 12.5
http://www.bhagavad-gita.us/articles/208/1/Bhagavad-Gita-125/Page1.html
By Bhagavan Sri Krishna
 

Text 5

kleso ’dhikataras tesam
avyaktasakta-cetasam
avyakta hi gatir duhkham
dehavadbhir avapyate

Translation

For those whose minds are attached to the unmanifested, impersonal feature of the Supreme, advancement is very troublesome. To make progress in that discipline is always difficult for those who are embodied.

Commentary by Srila Prabhupada 

The group of transcendentalists who follow the path of the inconceivable, unmanifested, impersonal feature of the Supreme Lord are called jnana-yogis, and persons who are in full Krishna consciousness, engaged in devotional service to the Lord, are called bhakti-yogis. Now, here the difference between jnana-yoga and bhakti-yoga is definitely expressed. The process of jnana-yoga, although ultimately bringing one to the same goal, is very troublesome, whereas the path of bhakti-yoga, the process of being in direct service to the Supreme Personality of Godhead, is easier and is natural for the embodied soul. The individual soul is embodied since time immemorial. It is very difficult for him to simply theoretically understand that he is not the body. 

Therefore, the bhakti-yogi accepts the Deity of Krishna as worshipable because there is some bodily conception fixed in the mind, which can thus be applied. Of course, worship of the Supreme Personality of Godhead in His form within the temple is not idol worship. There is evidence in the Vedic literature that worship may be saguna and nirguna—of the Supreme possessing or not possessing attributes. Worship of the Deity in the temple is saguna worship, for the Lord is represented by material qualities. But the form of the Lord, though represented by material qualities such as stone, wood or oil paint, is not actually material. That is the absolute nature of the Supreme Lord. 

A crude example may be given here. We may find some mailboxes on the street, and if we post our letters in those boxes, they will naturally go to their destination without difficulty. But any old box, or an imitation which we may find somewhere but which is not authorized by the post office, will not do the work. Similarly, God has an authorized representation in the Deity form, which is called arca-vigraha. This arca-vigraha is an incarnation of the Supreme Lord. God will accept service through that form. The Lord is omnipotent, all-powerful; therefore, by His incarnation as arca-vigraha He can accept the services of the devotee, just to make it convenient for the man in conditioned life. 

So for a devotee there is no difficulty in approaching the Supreme immediately and directly, but for those who are following the impersonal way to spiritual realization the path is difficult. They have to understand the unmanifested representation of the Supreme through such Vedic literatures as the Upanishads, and they have to learn the language, understand the nonperceptual feelings, and realize all these processes. This is not very easy for a common man. A person in Krishna consciousness, engaged in devotional service, simply by the guidance of the bona fide spiritual master, simply by offering regulative obeisances unto the Deity, simply by hearing the glories of the Lord, and simply by eating the remnants of foodstuffs offered to the Lord, realizes the Supreme Personality of Godhead very easily. There is no doubt that the impersonalists are unnecessarily taking a troublesome path with the risk of not realizing the Absolute Truth at the ultimate end. But the personalist, without any risk, trouble or difficulty, approaches the Supreme Personality directly. A similar passage appears in Srimad-Bhagavatam. It is stated there that if one ultimately has to surrender unto the Supreme Personality of Godhead (this surrendering process is called bhakti), but instead takes the trouble to understand what is Brahman and what is not Brahman and spends his whole life in that way, the result is simply troublesome. Therefore it is advised here that one should not take up this troublesome path of self-realization, because there is uncertainty in the ultimate result. 

A living entity is eternally an individual soul, and if he wants to merge into the spiritual whole, he may accomplish the realization of the eternal and knowledgeable aspects of his original nature, but the blissful portion is not realized. By the grace of some devotee, such a transcendentalist, highly learned in the process of jnana-yoga, may come to the point of bhakti-yoga, or devotional service. At that time, long practice in impersonalism also becomes a source of trouble, because he cannot give up the idea. Therefore an embodied soul is always in difficulty with the unmanifest, both at the time of practice and at the time of realization. Every living soul is partially independent, and one should know for certain that this unmanifested realization is against the nature of his spiritual blissful self. One should not take up this process. For every individual living entity the process of Krishna consciousness, which entails full engagement in devotional service, is the best way. If one wants to ignore this devotional service, there is the danger of turning to atheism. Thus the process of centering attention on the unmanifested, the inconceivable, which is beyond the approach of the senses, as already expressed in this verse, should never be encouraged at any time, especially in this age. It is not advised by Lord Krishna.

Commentary by Sri Visvanatha Cakravarti Thakur

This verse describes a little of their inferior position. Those who are attracted to only brahman (avyakta asakta cetasam), who desire only realization of brahman, receive excessive difficulties in attaining it, because (hi) by what means at all can something without qualities be made manifest? That goal is attained by the jivas with bodies (dehavadbhih) in such a way that there is suffering (duhkham). The senses have the power for particular knowledge such as sound, nor for something other than particularity. Therefore, stopping of the senses is necessary for those who desire knowledge of the impersonal without particulars (nirvisesa). But stopping senses is like stopping rivers. Stopping the senses is difficult. Sanat Kumara says:

yat-pada-pankaja-palasa-vilasa-bhaktya karmasayam grathitam udgrathayanti santah tadvan na rikta-matayo yatayo' pi ruddha-sroto-ganas tam aranam bhaja vasudevam

The devotees, who are always engaged in the service of the toes of the lotus feet of the Lord, can very easily overcome hard-knotted desires for fruitive activities. Because this is very difficult, the non-devotees—the jnanis and yogis—although trying to stop the waves of sense gratification, cannot do so. Therefore you are advised to engage in the devotional service of Krishna, the son of Vasudeva. SB 4.22.39

krcchro mahan iha bhavarnavam aplavesam sad-varga-nakram asukhena titirsanti tat tvam harer bhagavato bhajaniyam anghrim krtvodupam vyasanam uttara dustararnam

The ocean of nescience is very difficult to cross because it is infested with many dangerous sharks. Although those who are non-devotees undergo severe austerities and penances to cross that ocean, we recommend that you simply take shelter of the lotus feet of the Lord, which are like boats for crossing the ocean. Although the ocean is difficult to cross, by taking shelter of His lotus feet you will overcome all dangers. SB 4.22.40

Even that goal which is attained by such suffering is attained only by having a mixture with bhakti. Without bhakti to the Lord, the worshippers of the impersonal brahman obtain only suffering, and not brahman. Brahma says:

tesam asau klesala eva sisyate nanyad yatha sthula-tusavaghatinam

As a person who beats an empty husk of wheat cannot get grain, one who simply speculates cannot achieve self-realization. His only gain is trouble. SB 10.14.4