Just to confirm that we are on the same page, as I want to make sure that I understood your question well enough, what I’ll be trying to answer here is historical accuracy of events in Aristotle’s writing. Here, we’ll interpret the notion of history purely as historia, as an inquiry or investigation of past events, in a same manner Thucydides, Herodotus, Diodorus wrote about it. This may seem like I’m splitting hairs, but from one point of view, Aristotle is indeed a historian, he explored history of philosophy, which means that he documented and explored ideas and systems of thought, not events or political actions. There’s a lot to be said about his accuracy in philosophical interpretation of previous ideas (I’ll leave some sources at the bottom, for further reading, if wished*), but as you are interested in his reliability as a source of the time, we’ll concentrate on that.

As you mentioned, he has written a few passages about history, but there’s a relatively new and very valuable source that can be examined: The Constitution of the Athenians (Athenaion Politeia), which is his only work that can be purely observed as a historical, not philosophical one, and in which he describes the political system(s) of ancient Athens. When I say that it’s relatively new source, compared to his texts available to us from antiquity, this means that we only discovered it in 1879. It’s not part of the Corpus Aristotelicum (again, meaning the collection of Aristotle's works survived from antiquity through medieval manuscript transmission), and it’s been in the focus of the scientific community for only about 150 years. As it provided us with plenty of materials which were previously unknown (some historians claim that The Constitution of the Athenians was written and gathered as his research for Politics), there’s a claim that "the discovery of this treatise constitutes almost a new epoch in Greek historical study."^(1) However, most of historians who deep dived into research, criticized Aristotle’s ability as a historian and expressed doubt that he intended to write a real history (such as F. Jacoby and U. von Wilamowitz-Moellendorff). This critique, and this answers your question about bias, is based on the claim that the research Aristotle did for his Constitution was just an "a priori reconstruction based on categories developed in his Politics"^(2); which means that it was written as an theoretical alibi for his philosophical ponderings developed in Politics.

As your question was posed similarly by scholar Lucio Bertelli in his research (which I used today as well), I'll leave his quote instead of an conclusion, as well as a link at the bottom for his work:

I am inclined without hesitation to give a positive answer to the question with which I began: “Was Aristotle able to write history?” Aristotle’s history is a peculiar history based on a theory of scientific research and knowledge that we do not usually find in historians, but we cannot deny that Aristotle’s circumstantial processes are very similar to those of Thucydides.^(3)

On a parting note, if I may, I recommend finding version of "The Politics and the Constitution of Athens", edited by Stephen Everson (Cambridge Texts in the History of Political Thought, 1996), as it contains both of the works we mentioned, and shows both theoretical and empirical approach to Aristotle's understanding of political science (together with introduction, guides and notes which are golden).

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Further reading and resources:

*Stevenson, J. G. (1974) Aristotle as Historian of Philosophy. The Journal of Hellenic Studies, Vol. 94, pp. 138-143

*Guthrie, W. K. C. (1957) Aristotle as a Historian of Philosophy: Some Preliminaries. The Journal of Hellenic Studies, Vol. 77, Part 1, pp. 35-41

*McKeon, R. (1940) Plato and Aristotle as Historians: A Study of Method in the History of Ideas. Ethics, Vol. 51, No. 1, pp. 66-101

^(1)Mitchel, J. and M. Caspari, M. eds. (2001) A History of Greece: From the Time of Solon to 403 B.C., Routledge, p. xxvii

^(2 and 3) Bertelli, L. “Aristotle and History” in: Parmegianni, G. ed. (2014) Between Thucydides and Polybius: The Golden Age of Greek Historiography. Hellenic Studies Series 64, chpt. 13. (available here)