A very important parallel can be made with the transmission of ahadith; while the two extremes (authentic vs fabricated) have rulings which are clearly understood, there are also various grades in between, in which the authenticity can be neither guaranteed nor dismissed out of hand.
Excepting the ones that are truly weak, even ahadith which are not guaranteed to be authentic can be used to make authoritative rulings. And a number of scholars consider that even those weak hadith still have value; in the introduction to his Forty Hadith, An-Nawawi describes the majority scholarly opinion thus:
Religious scholars are agreed it is permissible to put into practice a weak Hadith if virtuous deeds are concerned…
It is important to note that classifying any hadith (or story) as weak is not the same as classifying it as false. Similarly, stories in which the authenticity is disputed — and as is often the case, impossible to verify — may (or may not) still be true. Such stories fall into the realm of al-Ghaib (the Unseen).
After narrating the story of Companions of the Cave in the Qur'an, God rebukes those who attempt to guess at al-Ghaib. Arguing about the details about any such story, including details such as whether it's authentic or fabricated, is futile unless clear evidence is available.
How unbelievable and fantastical it may appear is irrelevant. Belief in al-Ghaib includes believing in things that are otherwise unexplainable by science or observation.
Insofar as nobody was witness to the actual event, or has clear proofs thereof, narrating such stories as fact is (inadvertently) claiming knowledge of the unseen; this is arrogance tantamount to shirk. However, exactly the same argument can be made against outright decrying it as false. In order to claim that anyone should, or should not, believe and narrate such a story depends on its veracity; if its veracity is indeterminate, so too is any attempt at ruling on it.
The best one can do would be to apply the general ruling presented in the hadith of God's pasture: by only narrating those stories which are clearly permissible (such as those in the Qur'an, or those authentically narrated by the prophet, or those from the children of Israel) and avoiding all others, one can be sure they're not transgressing God's own limits, even if by accident.
However, it is incorrect to apply a general ruling such as "One must never tell such stories, nor believe in them," as there is no way of knowing that the stories themselves are not true. And so long as the following points are considered, I know of no evidences to outright forbid such:
- Such stories are not directly contradicted by clear evidences (such as the Qur'an),
- Such stories are not proclaimed as fact,
- There is a benefit to telling such stories (such as helping a person toward virtuous deeds)
But even then, avoiding doubtful matters is still preferable.