Difficulties?

There may be several reasons. Nichiren teaches:

"The Great Teacher T’ien-t’ai’s Great Concentration and Insight is the essence of his lifetime teachings and the heart of the whole spectrum of the Buddha’s sacred teachings. During the more than five hundred years after Buddhism was introduced to China, ten teachers appeared—seven in the north, three in the south. Their wisdom was as brilliant as the sun and moon, and their virtue was extolled throughout the country, yet they were confused as to which sutras were shallow or deep, inferior or superior, and as to the order in which they had been taught. It was the Great Teacher T’ien-t’ai Chih-che who not only clarified the Buddha’s teachings, but also brought forth the wish-granting jewel of a single moment of life comprising three thousand realms from the repository of the five characters of Myoho-renge-kyo and bestowed it on all people in the three countries. This teaching originated in China. Not even the scholars of India could put forward such a concept. So the Great Teacher Chang-an commented, “There has never been anything to compare to the brightness and serenity of concentration and insight,” and “Even the great scholars of India were not in a class with him.”

The doctrine of three thousand realms in a single moment of life revealed in the fifth volume of Great Concentration and Insight is especially profound. If you propagate it, devils will arise without fail. If they did not, there would be no way of knowing that this is the correct teaching. One passage from the same volume reads: “As practice progresses and understanding grows, the three obstacles and four devils emerge in confusing form, vying with one another to interfere... One should be neither influenced nor frightened by them. If one falls under their influence, one will be led into the paths of evil. If one is frightened by them, one will be prevented from practicing the correct teaching.” This statement not only applies to me, but also is a guide for my followers. Reverently make this teaching your own, and transmit it as an axiom of faith for future generations.

The three obstacles in this passage are the obstacle of earthly desires, the obstacle of karma, and the obstacle of retribution. The obstacle of earthly desires is the impediments to one’s practice that arise from greed, anger, foolishness, and the like; the obstacle of karma is the hindrances presented by one’s wife or children; and the obstacle of retribution is the hindrances caused by one’s sovereign or parents. Of the four devils, the workings of the devil king of the sixth heaven are of this last kind.

In Japan today, many people claim they have mastered the practice of concentration and insight. But is there anyone who has actually encountered the three obstacles and four devils? The statement “If one falls under their influence, one will be led into the paths of evil” indicates not only the three evil paths but also the worlds of human and heavenly beings, and in general, all of the nine worlds. Therefore, with the exception of the Lotus Sutra, all of the sutras—those of the Flower Garland, Āgama, Correct and Equal, and Wisdom periods, and the Nirvana and Mahāvairochana sutras—will lead people toward the paths of evil. Also, with the exception of the Tendai school, the adherents of the seven other schools28 are in reality wardens of hell who drive others toward the evil paths. Even in the Tendai school are found those who profess faith in the Lotus Sutra, yet actually lead others toward the pre-Lotus Sutra teachings. They, too, are wardens of hell who cause people to fall into the evil paths." - Letter to the Brothers

and

"However, there is a difference if one chants the daimoku while acting against the intent of this sutra. There are various stages in the practice of this sutra [and various forms of slander exist accordingly]. Let me sum them up by quoting from volume five of The Annotations on “The Words and Phrases of the Lotus Sutra”: “In defining the types of evil, The Words and Phrases of the Lotus Sutra states briefly, ‘Expound among the wise but not among the foolish.’ One scholar enumerates the types of evil as follows: ‘I will first list the evil causes and then their effects. There are fourteen evil causes: (1) arrogance, (2) negligence, (3) wrong views of the self, (4) shallow understanding, (5) attachment to earthly desires, (6) not understanding, (7) not believing, (8) scowling with knitted brows, (9) harboring doubts, (10) slandering, (11) despising, (12) hating, (13) envying, and (14) bearing grudges.’” Since these fourteen slanders apply equally to priesthood and laity, you must be on guard against them." The Fourteen Slanders or Letter to Matsuno. The Fourteeen Slanders are found in Chapter 3 of the Lotus Sutra, Simile and Parable