There is not a lot of ancient written material to be considered, certainly not in comparison to modern written material. For the entirety of ancient Rome, almost a thousand years of history, most of it encompassing a huge area and millions of people, the works of only a handful of historians survive. I should say: part of the works of those historians has survived, part is missing. Other historians who wrote in and about ancient Rome are known by name, but there are not many of them, either.
And not everyone who used to be considered an ancient historian is still thought of that way: the Augustan Histories, formally regarded as the work of six different authors writing around AD 300 and covering the reigns of the Roman Emperors from 117 to 284, is coming more and more to be regarded as the work of one author, writing around 400 and pretending to be six different earlier writers. And more and more, it is thought to have been written as something other than history -- as a satire of historical writing, perhaps. So what we used to think happened in the Roman Empire between 117 and 284 has to be re-considered to a very great extent. This agonizing re-appraisal is going on right now.
Not that the actual ancient historians are trusted completely. Far from it. They're regarded as themselves being entirely too trusting of written accounts of events of which they themselves were not eyewitnesses; they're suspected of twisting their historical accounts to serve their political agendas (perhaps contemporary historians are not suspected of this as much as they should be), and much of what they write is what we today would call historical fiction: for example, speeches and conversations with which the authors would have no way of being familiar are written out word-for-word, clearly invented by the authors, for what we would call dramatic purposes.
Because of the small amount and suspicious nature of the ancient historical writings, historians have no choice but to turn to other sources: ancient authors of non-historical works, including fictional and legendary works, are combed through for whatever tidbits of history reality they may contain; ancient coins and inscriptions are studied; the few surviving legal works and official versions of speeches of emperors are inspected.
And since the 19th century, the papyri from the eastern part of which have been unearthed, mostly written in Greek, found above all at Oxyrhynchus, besides ancient copies of Biblical and literary texts, some of which had been previously lost, have also added everyday items like personal letters, petitions, shopping lists and so forth.
However, the major sources remain those written by ancient historians. And besides wondering how far these historians themselves are to be trusted, there is the question of how accurately manuscripts of ancient authors reflect what those authors actually wrote. The attempt to re-construct as closely as possible what authors originally wrote is called textual criticism, and textual criticism is a very large and endlessly fascinating part of Classical Studies. In the case of Classical Greek, the above-mentioned discoveries of papyri have added a great deal of evidence with which textual critics can work. In the case of ancient Latin, recent discoveries have come much more seldom. In the case of most ancient Latin authors, there are no existing manuscripts older than the 9th century (Charlemagne, God bless him, instigated a huge revival of the study of ancient Latin). In some cases, there are no known manuscripts older than the 15th century (when printing began to replace manuscripts), and in the case of some authors, there are no manuscripts left at all: we have printed editions, but the manuscripts from which the earliest printed versions were made are gone. It is to be assumed, in the course of hundreds or thousands of years of copying and re-copying, some alterations to the texts were made.
And so, in the discussions which revolve around the textual criticism of ancient historians, there are debates which may look to the untrained observer as if they are debates about what exactly happened at a certain place and time, when actually they revolve around what a certain historian wrote, completely apart from the extent to which it is historically accurate. I saw these sorts of misunderstandings often in online discussions of Biblical texts, because, generally speaking, laypeople are much more interesting in discussing the Bible than in discussing any Classical authors: scholars, all atheists, none of whom believe in anything miraculous or otherwise supernatural, might be discussing, or trying to discuss, the best possible Hebrew or Greek version of a Bible passage, the version as close as possible to what the author actually wrote, while at the same virtual time and place, New Atheists and fundamentalists argue over whether or not the miracle describe in that passage actually occurred, and mostly ignore my attempts to tell them that the scholars were discussing, or trying to discuss, something entirely different.
Similarly, scholars might be discussing Vulgate manuscripts online, talking about whether the text of a particular manuscript showed that it was a copy, or a copy of a copy, of a manuscript made in a certain place and time, while constantly being interrupted by people asserting and disputing the literal historical accuracy of the Vulgate.
If you want to join a discussion, it's good to have a clue about what the people there are discussing: are they talking about what happened in a certain ancient time and place? Or what an ancient historian said about what happened in that time and place, or what we can infer from an ancient non-historical author? Or about what chance there is that the surviving manuscripts accurately what that historian (or other author) wrote? Or about what he or she may have written instead of what is in the manuscripts? Or about what the pattern of mistakes in manuscript A say about where and when the now-lost manuscript α was made, from which manuscript A was copied, or from which another now-lost manuscript, β, was copied, from which A was copied? Or one of many other topics which are not what happened at a certain time and place, but which may be a vital part of constructing a more accurate idea of what may have happened at that time and place?