In Christianity, the standard of fulfilled prophecy is 100%. In the Old Testament, this was so important that anyone who delivered a false prophecy was put to death. This is such a big deal that many false religions tend to abstain from prophecy. Mormonism, thankfully for Christian apologists, is not one of them.
In the above image, you see 3 failed prophecies (one of which remains in the Mormon scriptures to this day) and one completely bonkers statement, summed up with bullet points in the book Reasoning from the Scriptures with the Mormons by Ron Rhodes and Marian Bodine. Just these 4 demonstrably false statements are enough to categorically say that neither Joseph Smith nor Brigham Young can possibly have been prophets. But if Joseph Smith was not a prophet, Mormonism must be a false religion.