Every one of our eye witnesses tends to be dismissed as unreliable. So let's start by noting that all ancient historians are stigmatized as unreliable somewhere along the line. Our eye witnesses, as well as the historians who depend on them, should all be seen within that context.
The major cause for this comes from our perception of how history is done, which is quite different from the ancient one, and this in turn is a result of our understanding of what truth actually is—which is also quite different from the ancient understanding.
If we read ancient texts under modern assumptions, they will appear to be unreliable nonsense.
It is also worth remembering that the eye witness texts are more than 2000 years old.
They were copied many times by hand, and errors can creep in that way—especially where numbers and measurements are concerned.
Also, they were plagiarized by later historians (which was standard professional practice at that time), and blended with material similarly plagiarized from other sources.
So the original eye witness texts do not survive as they were written. We only have fragments scattered over many other people's work and we cannot be sure that they have been transcribed accurately. Tread carefully.