All information is useful, even deliberately falsified information, because it says something about the writer.

In one book I will cite below, the author thoroughly debunks the myths spread by the Jihadis of medieval Spain. Most of these writers were trash talking their Christian/pagan enemies, and buttering up to Islamic rulers who would dispense wealth or death on a whim. Even when the information is found to be false, it opens up questions as to if/why the author believed that, or why did he want others to believe it.

https://www.audible.com/pd/The-Myth-of-the-Andalusian-Paradise-Audiobook/B01HMRMIBM

Resolving discrepancies is a big part of research. More often than not, solutions can be found where most of the information is correct and the rest human error. Only a very bad researcher will dismiss a particular source or especially a whole category of sources for being unreliable. Even if that's true and the writer is a lying and ignorant, you can still learn something of the motives and intentions of the speaker himself.

An example of this is the Greek historian Thucydides wrote that Sparta stated they declared war on Athens for violating this treaty, but the real reason is they feared the growth of Athenian power and wanted to remain the biggest state on the bloc. Of course, Thucydides was an Athenian army officer, he probably believe the Spartans had ulterior motives but that probably wasn't true. There was surely lingering antagonisms due to earlier wars and outrage over Athens interventions. So while Thucydides stated a falsehood, he does reveal what the average Athenian was thinking (however flawed and biased that logic was).