I am under no illusion but that Mormonism is Incredible in the sense that it severely strains credibility, and simply seems ridiculous, absurd, disgusting to the standards of normal public discourse.
However, mainstream Christianity is incredible also, and in exactly the same way and by the same type of criteria (Incredible that is, to the external observer, who is neither mainstream nor Mormon) - which is to say: to accept any kind of Christianity is to accept the Incredible, and to reject Christianity is also to accept the Incredible.
Christianity is Incredible, Mormon Christianity is Incredible-squared.
Therefore neither mainstream Christianity nor Mormon Christianity should be presented as anything other than Incredible - because to do so is to misrepresent.
And because to do so is to diminish: positive transforming power is a consequence of Incredibility; so if our faith is in something less than the Incredible, then it is partial and enfeebled.
If Incredible things are to be rejected, then both Christianity and Mormon Christianity should be rejected.
But of course, the truth of Mormon Christianity is NOT entailed by the truth of mainstream Christianity - one can (the vast majority of Christians have and do) rationally embrace the Incredibility of mC while rejecting the incredibility of MC.
(The opposite is not possible - if someone believes that Mormonism is true then that does entail accepting the essential truth of mainstream Christianity.)
So it is reasonable, and indeed usual, for mainstream Christians to reject Mormon Christianity as Incredible-hence-untrue; however, they should be aware that precisely the same qualitative point can equally-reasonably be made against mainstream Christianity - if that Christianity is to be positive and transforming.
Only the Incredible can save us: only the Incredible would offer us everlasting life as Sons of God.
Our choice, here and now, is between Incredibilities - or despair.