Some cultures throughout history had written sources of history like Ancient Egypt, Rome, Greece, China, etc. But other cultures didn't have written traditions until much later(Native American cultures come to mind). Are those sources considered valid by modern historians given the fact that in many cases they are the only source of information we have when it comes to those histories?
This kind of depends on the historian in question. For me personally, I actually also have a very strong background in anthropology. Anthropologists have found oral traditions to actually be pretty “accurate” (or at least not less accurate than the also heavily biased written histories) when compared to other sources of information such as archaeology. As it turns out, the privileging of written records in earlier scholarship was pretty arbitrary and also Eurocentric. In my primary area of study, the Philippines, oral history is king in a lot of ways. The Philippines has indigenous writing systems going back around 1000 years but, despite the evident high level of literacy (for the times), the main method of keeping historical records and other kinds of information has been through oral traditions. Even now, it is very arguable that the Philippines is still to this day primarily a society that values oral traditions more than any other. These ways of keeping records are extremely important to me and to anyone else interested in social history of the islands.
I don't work with oral histories, but I have read several Indigenous anthropology books that suggest that some oral histories in Australia talk of migration into Australia (at least 60,000 years ago), the dramatic rising of sea levels and the extinction of the megafauna. In fact, the Rainbow Serpent of Australian mythology is often connected to a megafauna python that lived Australia wide.
Oral histories are also fairly useful in remote areas - many of Australia's frontier massacres were under or unreported in the news or in parliament, but were continually spoken about from generation to generation by Indigenous Australians. Often, it is these oral histories that push a historian to search for some obscure settler diary that also gives an account of the massacre - this was true of a place I visited called Kukenarup in southern Western Australia, where the Indigenous account was denied continuously up until the point where overwhelming evidence was found in written settler sources.
On this page, historian Lyndall Ryan talks about the different sources her team used to document massacres. It includes oral histories, which were often important due to the secretive and dishonest reporting of written sources, but they were never used without some form of written evidence supporting them. Thus, many massacres were left off of their frontier massacre map for a lack of written evidence so as not to have entire piece condemned as 'oral history fantasy'.