The "legitimacy" of religious prophecy is a question for certain types of theologians, not historians.

That said, the historical record is jam-packed with writers describing supernatural events or attributing supernatural causes o events. Medieval historical chronicles are filled with visions, people prophesying, devils, miracles--and the authors treat these events as "real" as the coronation of the next king.

Good historical questions are typically asked about two things: (1) Why the author said something like that (why do we see so many women claiming religious visions and prophecy in late medieval Europe?) and (2) the impact of that belief (when people read Joachim of Fiore's prophecies of the end of the world, how do they react? why do they react like that?).

It doesn't really tell us anything about the Middle Ages to say, "Hildegard of Bingen probably had migraines." It can tell us much more to investigate the question, "Why was a random nun, not even an abbess, able to become the most famous non-royal woman in the entire twelfth-century world?"