It definitely largely depends! For example, in the Roman Empire during Late Antiquity, there’s generally a lot of source information. Sources from this timeframe include church histories, hagiographies, panegyrics, etc. However, despite all these sources, there’s a lot of bias that comes with the period, especially between pagans & Christians. A lot of Pagan sources from this timeframe are missing or exist in fragments. The “best” source from this time period by Pagans is by a man called Zosimus. (His book is called the New History and I personally love his name). Historians think parts of his work are missing because of Christian suppression. In Zosimus’ case, the missing fragments could have possibly been about the treatment of pagans by Christians, and was suppressed due to negative views on them. Meaning, even though there are a lot of sources, it may not be as clear a view as one would think. There may only be a few biased sources, which don’t necessarily give the clearest view. For example, a popular telling of Diocletian becoming emperor portrays him as a reluctant leader nominated by a group of soldiers after the murder of the emperor by a prefect named Aper. There’s a lot wrong with that telling, and Stephen Mitchell does a really good job explaining: “This was the authorized version of events, and it is full of implausibilities. How had the murder of Numerian passed unnoticed? Why had Aper failed to seize the initiative that he had created by his crime? Why in any case had he allowed Diocletian to be acclaimed Augustus, if this is what he had plotted for himself? If Diocletian was genuinely a reluctant ruler, not responsible for overthrowing his predecessors, why should he have led an army against Carinus in Illyricum, instead of offering loyal support to him? Why then admit almost to losing the decisive battle and attribute his opponent's fall to his own moral degeneracy? A precise answer to all these questions is beyond our reach. Imperial coups d'état were not the subject of investigative journalism at the time, and the intrigues and plots which brought down or created emperors spawned self-interested rumor and accusations, not dispassionate enquiries. However, a reasonable reading of the evidence would acquit Aper of conspiracy and suggest that he had been the senior military officer who remained loyal to house of Carus, but had been killed by the new man whose cause had been promoted by a rival group of army officers. The logic of usurpation then determined Diocletian's attack on Carinus, while the story that he had been killed by one of his own officers diverted attention from the crime of the usurper to those alleged against the incumbent emperor.” Additionally, certain parts of the period have more information than others. There might be 3 books about one emperor and only fragments about the next. The period described as the Dark Ages is pretty much right after (at least when there was no Roman emperor in the West) and has very little source material. My own personal research is in colonial anthropology in the Philippines in the 1900s. Even though these sources were written relatively “recently,” most of my research is actually about how racial biases influenced colonialism and the anthropological studies done. Meaning, most of my work is about looking at sources written as science or fact, and figuring out the motivations behind the author and how it affected indigenous populations. One of the sources I look at heavily is a textbook, which would at first glance seem “truthful” but is very interesting to analyze otherwise! If you’re interested in Late Antiquity I cannot recommend enough the historian Peter Brown who popularized the era in the 70s in his book, The World of Late Antiquity.

Sources:

[Zosimus: New History, trans. Ronald Ridley (Canberra: Australian Association for Byzantine Studies, 1952) p. 50]

Mitchell, S. (2015). A history of the later Roman empire: Ad 284-641 (2nd ed.). Wiley Blackwell.