When I was young, I lived in a temple in the Japanese countryside which had a mantra as its main practice. The mantra is called Nembutsu, and the words are namu amida butsu, which means something like, "I call the name of Buddha." This is the chant used in Shin Buddhism, which is one of the two main chanting sects in Japan, the other being Nichiren. Recently, I was doing some internet browsing and came across a blog by a Nichiren Buddhist. In Nichiren, they chant the name of a sutra, and this writer said that for Nichiren Buddhists, Nembutsu was anathema.
I wasn't raised in a chanting environment, and I admittedly never really understood the power of Nembutsu, but I always assumed that mantras get their power from the intention of the believers who recite them. But maybe I've missed something. Maybe one also has to believe the chant is the one way to Buddha's heart. If that is the case, then one would feel obliged to vilify any other mantra that claims to have the same power.
This is a kind of fundamentalism, isn't it? I mean, the chant means nothing in itself and yet it comes to be considered a literal path to enlightenment, much like a literal reading of the Bible is considered requisite in fundamentalist Christian sects. I hadn't expected to find this in Buddhism.