The more I read about the Middle Ages now that I'm an adult, the more I learn that the whole time period from 500-1500 is more complicated than I was taught in school. It makes sense that the Vikings would be more complicated too.
The first is that most historians are not dispassionate observers concerned with recording literal truths; they are concerned with narratives containing moral truths. And, especially historically, lying about the facts to arrive at the moral truths was not beneath them. The history of Sparta is a classic example: our Greek sources here all talk up how wonderful Sparta is with its notions of equality and avoidance of luxury, and contain postscripts saying "but of course, this is the Sparta of yesteryear and not the Sparta of today, which is exactly the kind of wastrel that I'm condemning." I do want to be clear that this kind of moralism-over-literal-truth does happen all the time today, and it's definitely visibly strong in the textbook version of history taught today. Consider how US history courses at the grade school level tend to present the US as a moralizing force of liberty and democracy and manage to completely omit traumatic events such as King Philip's War, the deadliest war ever fought on (then-future) US soil, perhaps partly because it is rather inconvenient for the narrative of pious Pilgrims peacefully coexisting with Native Americans.
Specific to the Middle Ages, though, the humanism and Enlightenment movement that happens after that time did try quite hard to discredit any idea of progress in that time period and draw push their modernism as a rebirth of the glory of Rome, Greece, and the Classical Age. Like most historians, they had a particular narrative of the Middle Ages that mattered more than the truth, and they pushed it hard. And subsequent historians repeat that narrative verbatim without considering if perhaps their sources might not be telling the truth. Some of our notions about how the Middle Ages works actually applies much better to the Early Modern period instead--for example, it was during the Early Modern that bathing was restricted because "modern science" (the miasma theory of disease) knows that bathing is how you get sick.
A final point is that there is a particular axe to grind with the question of religion. Many Protestants sought to deny anything that the Catholic pope did. The Middle Ages, being an era where the Catholic Church and the pope had far greater political power than he holds today, then becomes a prime target of criticism. Indeed, this criticism did reach the point that the Christianization of the Roman Empire was blamed for its fall (cf. Edward Gibbon).
I often comment that to learn history, you should first forget everything you learned in school about it. It feels to me that it's more often wrong than right.
https://acoup.blog/2019/08/16/collections-this-isnt-sparta-p...
* Really, the time where there weren't any written records were mostly concentrated to 500-750 or so. Even though I had learned about Charlegmagne in school, I hadn't really learned much about all he did to encourage reading and writing (obviously, to the extent that it was possible given the cost of vellum at the time). The amount of written records were also not uniform geographically. As the original article says, Scandinavians wrote less than people in souther Europe.
* There actually are decent-for-pre-modern records of England from the time around the Vikings. The story in school was mostly "Things were great under the Roman empire, then things fell off a cliff and stayed that way for 1000 years." In reality though, there are the Domesday Book or the writings of Venerable Bede (just to mention two examples) that record events of the time.
Of course, there wasn't the printing press so it is technically true that things were less documented, I think this is just less so than commonly believed.