According to Islam, Muhammad received revelations from the Angel Gabriel. Let's grant that for the sake of argument.
According to Muslim tradition, Muhammad was illiterate. So he dictated his revelations to scribes.
This wouldn't be the same as transcribing a revelation direct from the lips of Gabriel. For Gabriel didn't appear to Muhammad's scribes; rather, he appeared to Muhammad.
At best, therefore, this would be a case of Muhammad dictating to a scribe what he remembered Gabriel telling him. Unless he had verbatim recall of what Gabriel told him, this will be a paraphrase of what he heard. We typically remember the gist of what people say, not the exact wording.
In addition, did Muhammad have scribes at his beck and call from the time Gabriel first appeared to him? Or did he only have scribes after he acquired a significant following?
If the latter, then he didn't dictate his revelations when they were fresh in his mind. Rather, years might have elapsed, with a backlog of revelations, before he had scribes to whom he could dictate what, to the best of his recollection, Gabriel told him.
In that event, the earliest revelations would be in the poorest state of preservation, since they'd depend on Muhammad's memory years later.
And this is apart from the state of the unfinished Koran upon his death.
Since it's literally a dictation theory of inspiration, it's only as good as Muhammad's natural fallible memory.
That's even assuming we grant the claim that the Gabriel periodically appeared to him and spoke to him.