I will provide a hint to your question grossly, since I'm running out of time. Provided chance, I shall attend to this question if needed.
Some Pure Land masters are popularly quoted as saying that even the unrepentant and evil gain access to Sukhāvatī via niànfó in this very life in which they are criminals or evil men.
If you understood the Pureland doctrines thoroughly, you should learn that there are 9 grades for entering Sukhāvatī, called 九品蓮花 (Nine Platforms of the Lotus [transportation vehicles?!]): best, better, good x 3 = 9). The lowest of these 9, i.e., the good of the good (下品下生), is that when a practitioner who's committed incl. the Five Grave Sins (五逆罪) such as killing the parent, by the time of dying is able to remember/ think of (憶念) Amitābha he will be born in Sukhāvatī. However, the Lotus Platform that carried him takes 12 Great Kalpas to open, i.e., he will arrive in Sukhāvatī after 1612800 million years(?) [161.28 亿年]. The Best of the Best takes a blink of eyes. The criteria must be met is that he should be able to remember/ think of Amitabha - top importance; he repents/laments his wrong deed (懺悔) though he couldn't mend it; he himself has the full confidence of being able to be born in Sukhāvatī.
If any teacher just stating that Niànfó is the one-fix-all, without explaining the above, he is misleading and that's why Pureland to outsiders, even insiders, are sometimes criticized rigorously. I knew there is one very popular and famous Pureland School's Bhikkhu living in Singapore migrated from Mainland China who's teaching an abridged version of the doctrine [his own doctrine?]. He is also called into question by Buddhists of if his meeting the qualification of being ordained as a Bhikkhu because in one occasion in defending himself he revealed that he was born incompetent (黃門), this video with his voice is circulating in the internet. I hope you are not the students following his teaching ;).
If you want to learn properly, I suggest you read the original Classical Chinese 《佛說無量壽經》 《佛說觀無量壽經》, 《佛說阿彌陀經》 is translated by Kumarajiva; or letters of 印光大師. They are not that hard to read, if you know Chinese.
Also, for Pureland practice, Niànfó is just one of the many methods. Such as above Sutras, it is in fact in it recorded the visualization method, the 《佛說觀無量壽經》 there are in total 16 visualizations one of it is visualizing the setting sun as described in the Sutra.
But the sad thing is, practitioners just follow the teachers, and the methods reduced to one single Niànfó, it is not sure the teachers if they studied the Sutras, or just busy writing articles giving talks to invoke more audiences to recognize their names/fames. No one care to learn from the original Sutras, from directly the Buddha himself.