I'm fine if they compress a little bit, simplify story and shorten character list for story telling reasons. It is unreasonable to include all events and people affecting to some historical happenings. Also, the real life never creates a clear story line with beginning, middle and ending, which are pretty much required for a good story structure.
But yes, they should stick to truth and realism if they are going to tell about historical events or claim to describe historical times. Even thought King Arthur and the Knights of the Round Table are very deep into supernatural fantasy, their historical depiction should be at least consistent with the selected historical times (tribal post-Rome period, feudal high medieval setting...)
While I don't lose my night's sleep for inaccuracies, I still don't like to realize when they have forcefully applied modern customs, values and ideologies to a history. For example, medieval times: No, uprisings were not attempts to replace despotic rulers with democratic and both socially and ethnically equal societies. No, women were not generally allowed to argue against men (any men, not only husbands) without consequences. No, women did not use strong eye liner and bright lipstick (they did try to look healthy and less suntanned than field-working commoners, though). No, there was not much of foreigners of different distinct ethnic and especially religious background among commoners (foreign trade hubs, and nobility and their direct servants are different thing). No, such foreigners were not treated equally, and hardly with acceptance, among commoners even if they had adopted Christianity. No, commoners were not allowed to travel freely between feudal provinces. No, this wasn't a time of witch trials, and even when there was a trial, the church was skeptical about such accusations. No, black fitted leather suits were not popular clothing, even Vikings wore rather colorful, fancy and even dandy clothing.
But are wrong facts harmful? Twisted facts about history are constantly used to justify intolerant and such actions. "This land belongs to us", "this land has always been inhabited by our kind", "this ethnic minority / majority has always oppressed us", "we didn't do anything wrong, it was always only them".