To some extent, documentaries are a reliable source of information from an educational perspective. However, the more sensitive the topic becomes the likelier some parts of it may get altered. Then there's also the minimal bias of the filmmaker and the language barrier regardless of translations.
Also, keep in mind that documentaries are narratives. Educational narratives, yes, but still stories all the same. The filmmakers have to make money, after all. A filmmaker can spin events, so for that reason I don't trust documentaries completely. I find documentaries interesting and educational, but I also take it with the tiniest grain of salt.
@SushiRoll has a point with it being a narrative and there's also the marketing side of it. It would be better to watch multiple documentaries about a single topic if the viewer is concerned with the truthfulness of the details. Because in History, for example, the narrator would have a deep influence on what would be highlighted as important while other events get overlooked.
It depends on who funded the project, who stands to gain from it, and the intent behind the filmmaker, director, and those who agreed to take part in the documentary. "There is your truth and there is my truth. As for the universal truth, it does not exist.” It's a great quote by Amish Tripathi that kind of sums up how I think about it.