In general, I would argue that sects do not really have a major effect on religious pluralism. By that, I mean that the rise of one sect or another does not qualitatively change religious pluralism in a country. Instead, the rise of various sects can only add to religious pluralism in the country and change the details of the range of beliefs encompassed by that pluralism.
I will look at three “uniquely American sects.” Two of these came out of the Second Great Awakening in the early 1800s and one is much more modern. The two sects that arose in the 1800s are the Church of Jesus Christ of Latter-Day Saints (Mormons) and the Seventh-Day Adventists. Both of these sects came out of the “burned over district” of upstate New York. The SDA church originated with William Miller, an apocalyptic revivalist who predicted that the second coming of Jesus Christ would occur in 1843. When it did not, some of his followers altered their understanding of the prophecies that Miller had based his prediction on. These followers founded the SDA church. The LDS Church was founded by Joseph Smith in 1830. He based the church’s organization and doctrines on what he said were ancient scriptures that he had found near his home in upstate New York. The Mormons quickly became unpopular among their neighbors, in part for their practice of polygamy (which is no longer a Mormon practice). This led eventually to the murder of Smith and the exodus of the Mormons which ended when they reached the area of what is now Salt Lake City, Utah.
The modern sect that is uniquely American is the Nation of Islam. This church is, of course, based on Islam, which is not a uniquely American faith. However, the NOI espouses doctrines that separate it from mainstream international Islam. Essentially, the NOI is a black nationalist faith whose teachings concentrate on issues that have to do with the problems of race in the United States.
These uniquely American sects contribute to American religious pluralism to this day. The SDA church and the NOI are relatively small, but the LDS church is the fastest-growing church in the world. However, we cannot really say that these sects changed American religious pluralism in any significant way. Instead, they simply made the American religious scene more diverse.