There are many things one needs to consider when he is analysing the historians of the eras you are talking about and it's important to understand that many times the models have reliability issues due to lack of facts, knowledge coming from oral traditions and perceptions of the time, mainly based on the religious systems. Nonetheless, in the example you've given about these particular stories and their lack of value-free judgement, it's also important to take into account the personalities of the historians. For example, we have seen many instances of cynical view from Tacitus, who claimed that he even knew what the emperors were thinking when they told otherwise or praising the people of Germania, the enemies of the Roman empire. There are some cases in history that I believe that the personal input of a historian has to be applied but generally, the more objective the historian is and the more interpretations he provides, the more successful will his work be. And to add a point to your statement about being most critical towards Nero, I disagree and having read "Agricola" multiple times I believe that he was more critical towards emperor Domitian, who was even accused of giving the order to murder Tacitus' father-in-law, Agricola, due to him gaining popularity, power and prestige.