Scriptures are authentic, there is no doubt in it. But it is also true that some stories in them can be fictional. Scriptures like the primary puranas and other texts were written not by ordinary humans, but by sages and saints who were able to know the past present and future. And it was neither the nature, nor the duty of the sages to lie, bewilder and mislead the humans. So whatever stories are there which seems fictional, are either have esoteric meanings that we don't understand or they are there for the greater good of explaining certain concepts and give knowledge in a story form. It is a commonly known fact that stories are an effective method of imparting knowledge. So the Yoga Vashishtha says about this in a similar context:
O Rama, without an illustration, truth that has not be personally experienced, cannot be grasped just like without the light objects in the house cannot be seen in the night. [YV - 2.18.51]
Secondly, they are fictional in the sense that the sages were poets by nature and whenever necessary they used their poetic imagination to describe events and incidents more vividly and beautifully with ornamental language. But to tell and believe that the incidents never happened at all would be foolishness because that would be accusing the sages as liars. Only when one meets a sage that one can know how powerful they are, otherwise he will only be speculating just like a nice guy who once told me how could Vedavyasa know and write so many things in details if those were not his own imaginations? So all these are myths, that's why they are called mythology. But if they were all myths, how could Vedavyasa write the actual position of stars in the sky, planetary positions, etc. which have already been verified by researchers?
So thinking them fictional, undermining and neglecting them is only our lack of understanding. However, if we get the message or the esoteric meaning of the stories, then there is no problem in considering the stories as fiction. But if one loses faith thinking them to be fictions only, then he will cause his own degradation owning to the lack of faith in the sage's words. So the Gita says:
ajnas casraddadhanas ca samsayatma vinasyati
nayam loko 'sti na paro na sukham samsayatmanah [BG - 4.40]
But ignorant and faithless persons who doubt the revealed scriptures do not attain God consciousness. For the doubting soul there is happiness neither in this world nor in the next.