I regard the Islamic textual tradition as belonging to the same category as the Christian (and both Buddhist and Hindi) and thus unreliable. There is a view today that this religion came into being after the Arab Conquest of Jerusalem:
Islam: The Untold Story is a documentary film written and presented by the English novelist and popular historian Tom Holland. The documentary explores the origins of Islam, an Abrahamic religion that developed in Arabia in the 7th century; it criticizes the orthodox Islamic account of this history, claiming that it lacks sufficient supporting evidence.
Adopting the controversial theories of academic historian Patricia Crone as a basis, Holland asserted that there was little hard evidence for the origins of Islam and asked why it took several decades after the death of Muhammad for his name to appear on surviving documents or artifacts.
This religion's most sacred text:
The Sana'a palimpsest, dubbed Ṣanʿā’ 1, is one of the oldest Qur'anic manuscripts in existence. It was found, along with many other Qur'anic and non-Qur'anic fragments, in Yemen in 1972 during restoration of the Great Mosque of Sana'a. The manuscript is written on parchment, and comprises two layers of text (see palimpsest). The upper text conforms to the standard 'Uthmanic Qur'an, whereas the lower text contains many variants to the standard text. An edition of the lower text was published in 2012. A radiocarbon analysis has dated the parchment containing the lower text to before 671 AD with a 99% accuracy.
There is even an argument that the language is not originally Arabic, but Syriac. Slaves, perhaps.
In short, we do not know for sure what is the history of the early Arab Conquests, nor of the Koran, nor even what the original text contained (or who wrote it). [Personally, I find intriguing how Arabia was mainly Jewish at the start of the 7th century (the south was a Jewish kingdom and some tribes in the centre had converted).]
To be able to check what these Arabs had to say - at the time - about the religions of the Eastern Roman Empire, Italy (and Rome), and Hispania, as they found them, would be fascinating. Not sure at the moment such records exist, or ever existed. I know histories were written after, but how sure can we be that they are more reliable than those of the Christian textual tradition?
I include texts as artefacts, because IMO that is what they are; many also contain cultural layers, the study of which defines archaeology.
My approach is to allow artefacts of a later period to be inserted into an earlier only when a strong argument can be made for doing so. Take the texts attributed - as an example - to one "Augustine of Hippo": nobody knows who wrote (supposedly copied) them, where, or under whose authority; I know of no scholar who has ever asked such a question. Perhaps I am the first?