You're letting your modern biases color how you view Tacitus' writings. Most people educated in the last ~hundred years or so were taught "history" as you understand it - an impartial account of the facts of an actual event or person. This view of history is actually pretty recent, and it's widely understood that ancient historians did _not_ practice what we would consider the modern discipline of history. For example, Herodotus is considered the historian ne plus ultra of the ancient world, but he still wrote about lots of weird shit like zombies and races of headless people.
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).
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).