There are several Japanese Buddhist sects that have fallen into Founder Worship. Within Nichiren Buddhism, the Taisekiji branch and it's offshoots have taken it up. This reflects a desire of some Japanese people to elevate their countrymen in status and to discard or demote anything not Japanese. This is the root source of the Fuji school's (SGI and NST) "Nichiren Hombutsuron" [Nichiren as True and Original Buddha] doctrine.
Founder Worship is a useful theology to employ if you are not particularly loyal to the teachings of the Patriarch, by giving the appearance of 'super loyalty' by making him personally the Honzon (the Principal to be venerated). This is not an authentic form of Nichiren Buddhism. Nichiren continually exhorted his followers to; "Follow the Dharma, not the Person."
"I, Nichiren have, indeed, written it by letting my soul (tamashi) dipped in ink flow forth"
He was indicating the importance of the Mandala, not vaguely hinting at some sort of "soul transference" into the Mandala. The exact word he used was"Tamashi". This indicated his oneness with the Object and that he was putting his full effort and idea into it. Another example would be when a Samurai puts his "Tamashi" into his sword. Taisekiji metaphysics needs to warp passages like this in order to find hints at their home made doctrines that do not actually exist in the Gosho when read in a normal manner.
Rejection of this Founder Worship model by no means is a rejection of Nichiren's Enlightenment. He surely manifested 'Soku Shin Jobutsu' (Enlightenment in this body) and revealed his most inner realizations in his writings while on Sado Island and later on Mt. Minobu. Here is what he wrote in the "Kaimoku Sho" on Sado Island while he was still uncertain of his survival. He called this text "his last will" if he should perish. It completely defeats (and even scolds) the Taiskeiji heresy. Here it is:
From the "Kai Moku Sho", written on Sado Island:
"The Kegon and Shingon Schools established Vairocana and the Great Sun Buddha instead of Sakyamuni Buddha as the object of worship respectively. It is like a king's son despising his father while respecting a nameless person who acts as though he were the King of the Dharma."
"The Pure Land School considers Amitabha Buddha, who is merely a manifestation of Sakyamuni in the Pure Land to the West, to be the lord of this world and abandons Sakyamuni, who is the real lord of this world. The Zen School, just like a lowly man with little virtue despising his parents, despises the Buddha and His sutras."
"They are all confused about the main object of worship. It is analogous to the situation before the time of the Three Emperors, when people did not know who their fathers were and behaved like beasts and birds. Those Buddhist schools which do not know the "The Life Span of the Buddha" chapter are the same as beasts and birds; they do not know the debt they owe to the Eternal Buddha. Grand Master Miao-le declared; Eternity in the life of the Buddha, who is our parent, has not been revealed in the pre-Lotus sutras. If one does not know the eternity in the life of his father, the Buddha, he is also confused about the Buddha land his father governs. Such a man, no matter how capable he may be, is not at all worthy of a human."
"Grand Master Miao-le was a man of the T'ien-pao Period toward the end of the T'ang China. Having widely read and deeply contemplated canons of the Sanlun (Sanron), Hua-yen (Kegon), Fa-hsiang (Hosso), and Chen-yen (Shingon) Schools, he concluded that those who do not know the Original Buddha revealed in the"The Life Span of the Buddha" chapter are like "beasts and birds", who may be talented but do not know of the land governed by the father. "No matter how capable he may be" refers to men like Fa-ts'ang and Ch'eng-kuan of the Hua-yen(Kegon) School and Venerable Subhakarasimha (Shan-wu-wei) of the Chen-yen (Shingon) School, who were men of talent but, like children who do not know their father, did not know of the true and eternal Sakyamuni Buddha."