First, here is the element of Postmodern discourse I'm talking about:
https://en.m.wikipedia.org/wiki/Deconstruction
Second, my complaint is that it's not even considered as a possibility that Tacitus is telling the truth. In relation to The Annals (a public document concerning public affairs thousands of contemporaries would have had firsthand experiences of) do you really think it's a forgery? Or do you really think Tacitus is so biased he can't record history accurately? The idea that someone becomes so corrupted by their interests (especially race and class) that they cant be objective is a uniquely postmodern idea. The Enlightenment was premised on the idea that objective rationality was accessible to all.
Further, we can test your hypothesis about the Renaissance. The Renaissance writers read Tacitus, and when they mention him, they treat his works like a complete and unmitigated factual account. Also, Tacitus is not engaged in rhetoric. That's the point. He's writing history, not philosophy.
It's not so much that modern historians think Tacitus was too "biased" to "record history accurately"; they read his works critically because they know he wasn't really even trying to record history accurately in the way we think about doing that today.
EDIT: Another good example of ancient versus modern history is Pericles' Funeral Oration, as related to us by Thucydides: https://en.wikipedia.org/wiki/Pericles%27_Funeral_Oration Thucydides probably edited the speech heavily (even by adding or removing content), and may have even combined multiple different speeches to create what we know as the speech. A modern historian would most likely blanch at the thought of doing this, but Thucydides was fine with it because he wasn't even trying to relay an impartial and 100% accurate account of the events of the Peloponnesian War (as a modern historian would).
There is a 500 year difference bt Herodotus and Tacitus. Even so, Herodotus did no more than accurately record what he was told, assiduously pointing out when he saw something first hand. Early ancient historians made up speeches, that much is known. But by Tacitus's day, the act of writing an objective history was not a novelty, and in fact he complains in the beginning of his text that he is undertaking the work because he thinks his peers, also ostensibly engaged in objective history, have not been objective enough out of fear or hatred when covering the reigns of Tiberius, Caligula, Claudius, and Nero. Once again, with no reason to doubt him, I ask: why not even consider taking one of the greatest historians whose works have been preserved for posterity at face value?
Is there anything in particular Tacitus says in any of his works that you doubt happened?