CC Madhya 6.168

veda nā māniyā bauddha haya ta’ nāstika
vedāśraya nāstikya-vāda bauddhake adhika
Word for word: 
veda — the Vedic literature; — not; māniyā — accepting; bauddha — the Buddhists; haya — are; ta’ — indeed; nāstika — agnostics; veda-āśraya — taking shelter of Vedic civilization; nāstikya-vāda — agnosticism; bauddhake — even Buddhists; adhika — surpassing.
Translation: 
“The Buddhists do not recognize the authority of the Vedas; therefore they are considered agnostics. However, those who have taken shelter of the Vedic scriptures yet preach agnosticism in accordance with the Māyāvāda philosophy are certainly more dangerous than the Buddhists.
Purport: 

Although the Buddhists are directly opposed to Vaiṣṇava philosophy, it can easily be understood that the Śaṅkarites are more dangerous because they accept the authority of the Vedas yet act contrary to Vedic instruction. Vedāśraya nāstikya-vāda means “agnosticism under the shelter of Vedic culture” and refers to the monistic philosophy of the Māyāvādīs. Lord Buddha abandoned the authority of the Vedic literature and therefore rejected the ritualistic ceremonies and sacrifices recommended in the Vedas. His nirvāṇa philosophy means stopping all material activities. Lord Buddha did not recognize the presence of transcendental forms and spiritual activities beyond the material world. He simply described voidism beyond this material existence. The Māyāvādī philosophers offer lip service to Vedic authority but try to escape the Vedic ritualistic ceremonies. They concoct some idea of a transcendental position and call themselves Nārāyaṇa, or God. However, God’s position is completely different from their concoction. Such Māyāvādī philosophers consider themselves above the influence of karma-kāṇḍa (fruitive activities and their reactions). For them, the spiritual world is equated with the Buddhist voidism. There is very little difference between impersonalism and voidism. Voidism can be directly understood, but the impersonalism enunciated by Māyāvādī philosophers is not very easily understandable. Of course, Māyāvādī philosophers accept a spiritual existence, but they do not know about the spiritual world and spiritual beings. According to Śrīmad-Bhāgavatam (10.2.32):

ye ’nye ’ravindākṣa vimukta-māninas
tvayy asta-bhāvād aviśuddha-buddhayaḥ
āruhya kṛcchreṇa paraṁ padaṁ tataḥ
patanty adho ’nādṛta-yuṣmad-aṅghrayaḥ

The intelligence of the Māyāvādīs is not purified; therefore even though they practice austerities for self-realization, they cannot remain within the impersonal brahmajyoti. Consequently, they fall down again into this material world.

The Māyāvādīs’ conception of spiritual existence is almost identical to the negation of material existence. The Māyāvādīs believe that there is nothing positive in spiritual life. As a result, they cannot understand devotional service or the worship of the Supreme Person, sac-cid-ānanda-vigraha. The Māyāvādī philosophers consider Deity worship in devotional service to be pratibimba-vāda, or the worship of a form that is the reflection of a false material form. Thus the Lord’s transcendental form, which is eternally blissful and full of knowledge, is unknown to Māyāvādī philosophers. Although the term “Bhagavān” is explicitly described in Śrīmad-Bhāgavatam, they cannot understand it. Brahmeti paramātmeti bhagavān iti śabdyate: “The Absolute Truth is called Brahman, Paramātmā and Bhagavān.” (Bhāg. 1.2.11) The Māyāvādīs try to understand Brahman only, or, at the most, Paramātmā. However, they are unable to understand Bhagavān. Therefore the Supreme Personality of Godhead, Kṛṣṇa, says, māyayāpahṛta-jñānāḥ. Because of the temperament of the Māyāvādī philosophers, real knowledge is taken from them. Because they cannot receive the mercy of the Lord, they will always be bewildered by His transcendental form. Impersonal philosophy destroys the three phases of knowledge — jñāna, jñeya and jñātā. As soon as one speaks of knowledge, there must be a person who is the knower, the knowledge itself and the object of knowledge. Māyāvāda philosophy combines these three categories; therefore the Māyāvādīs cannot understand how the spiritual potencies of the Supreme Personality of Godhead act. Because of their poor fund of knowledge, they cannot understand the distinction in the spiritual world between knowledge, the knower and the object of knowledge. Because of this, Śrī Caitanya Mahāprabhu considers the Māyāvādī philosophers more dangerous than the Buddhists.