I took part in this interesting exchange on a webforum:
Q: "Traditional Gauḍīya Gurus have said that one should not take prasādam at the Iskcon/GM temples, This does not come from just one traditional Gauḍīya Guru alone, but is a general consensus it seems. It is also said by traditional Gauḍīyas that Rādhā-Kṛṣṇa are not present in the dieties of Iskcon/GM, reason being because they do not have proper dīkṣā in those institutions, so therefore the dieties are not properly installed without that proper channel."
Advaitadas: "These are two separate points really. For the first issue, my Sādhu Bābā indeed advised me not to eat outside our āśram, though he did not aim that at Iskcon alone. For the second point, I'd like to quote my own blog of February 27, 2006:
"In a private darshan I ask Satyanarayan whether Kṛṣṇa will not be there in the deity because of corrupt management of its temple or org, due to lack of prāṇa pratiṣṭā or lack of bhakti. He says prāṇa pratiṣṭā is just a show conducted for social acceptance of a temple. Kṛṣṇa is anyway everywhere, so why not in a non-installed deity? Kṛṣṇa is manifest in the deity according to the bhakti of the individual who comes for His darśan."
Satya-nārāyan Bābā is initiated in an ancient line, from Gadādhara Pandit, so it is not that there is a consensus on this among traditionalists."
Q: "So let me ask you advaita dasji.... Do we really even need diksha for diksha mantras to manifest their potency...or is that just an unnecessary external ritual?"
Advaitadas: "Dīkṣā is prescribed in Bhakti Rasāmṛta Sindhu, Bhakti Sandarbha and Haribhakti Vilāsa, so there is no doubt about its necessity. The starting theme of this thread was whether God is present in the deity according to the dīkṣā of the pūjārī. I certainly agree with Satya-nārāyan's opinion here that He is there in the eye of the beholder (devotee). Muslims consider it their sacred duty to destroy the deity because they see it as an insult to God to limit Him to a statue. That destruction is the greatest offence imaginable for us, but for them it is an act of devotion to that very same God. The point is, it is the attitude towards the deity that counts. For example, the deities in Iskcon Vṛndāvan have been installed by Rādhāraman Gosvāmīs, who received dīkṣā in Gopāl Bhaṭṭa Gosvāmī's line, and worshiped by followers of Siddhānta Sarasvatī, whose lineage is in doubt (it is not even sure whether he really did not receive dīkṣā, and AC Bhaktivedānta Swāmī had a family guru at the age of 12, so he has mantra in paramparā, though we do not know in which line) - now what is it going to be? God is there in the deity or not? Is the deity like a traffic light that goes on and off according to the (initiated/non-initiated) person who worships Him? Is there any evidence for that philosophy that God is not present in the deity because one or two pūjārīs MAY not have proper dīkṣā? Please consider all this. It doesn't add up."