Master Chi Yun Guan Ting (empowered) had said in the commentary of The Sutra of Visualization of the Buddha of Infinite Life, that of ALL the method/sutra/mantra that cannot rid the heaviest karma, there's still this method of Nienfo(Buddha remembrance). And no ordinary people can said this word without first understanding the Dharma. Bodhisattva Mahastamaprapta had taught us to shut all our 6 faculties, and use a puremind to recite unceasingly without doubt, without mixture. If there's mixture, puremind and faith would not be 100%, so to speak. Example an old praxis used to recite, Diamond sutra, Ksitigarbha Sutra, Medicine sutra, etc, after hearing the goodness of Nienfo method, now stop reciting them, would tends to fear a lose. They might think if they don't recite Medicine sutra, Medicine sutra would not help when they get sick, etcs. Seems like they had divided Buddhas bodhisattvas into categories, and all must recite a bit and cannot offense either one. This is call mixture and doubt. With this mind to do Nienfo, it's not going to work say MCK. We need a puremind to Nienfo, single-mindedly, rather than double-mindedly. So Masters said, Non-doubt; non-mixture and non-stop reciting is the key.
"Enlightenment is to turn around and see MY own mistake, Other's mistake is also my mistake. Others are right even if they are wrong. i'm wrong even if i'm right. " - Master Chin Kung