For how long? I don’t think it was much more than a century, although it’s hard to pin down. Meanwhile, Mormonism is almost 200. Some science fiction event could change things, sure. But the current trend seems to be that Mormons cannot stop telling everyone that they’re Christians, too. Unless it’s case of “the lady doth protest too much.”
Doctrine: I know fundies hate lots of Mormon beliefs, along with the other ones you mentioned, primarily over Trinitarianism (I think Adventists or SDA belong in that group, though). Also many classic groups like Arians, who were considered Christian if heretical. Mormons are “different,” but they have company.
Jews for Jesus are Christian though, despite their protestations.
Aside: classification is kind of odd. Like: are Anglicans Protestant? They broke off from the Catholic church. But the English resisted Luther’s pull for true Protestantism, and only left over Henry VIII’s penis. Also, the practices in the high church is closer to Catholicism. But then there’s the Franciscans: Catholic as you can get, but they exist in Anglicanism and even Lutheranism still. Protestantism is a catch-all for not-Catholic, not-Orthodox usually.
Anyone want to place a bet on Scientology getting in on the action? They already use a cross ripoff. I don’t know, but I’ll hazard that their stated symbolism is only part of the story, and it’s intended to be more palatable to Christians. And they claim you can be both…