Well, considering this is Nero he's talking about, its pretty easy to sympathize with pretty much anyone or anything else, in comparison :v
Nero was never very popular in Rome, so its natural that contemporary Roman historians and scholars would find reasons to portray him in a negative light regardless of what they were discussing. Hell, from all we know, most sordid/nefarious details of Nero's personal life (incestous relationship with his mother, dressing his catamite as his late wife and calling him by her name while crying on his lap, running down children under his chariot, etc) might just be political propaganda fabricated by his many contemporary detractors.
And in regards to the survey thing, pretty much what everyone said. There are contemporary and multiple sources (some reliable, other less so) pointing to the figure of Jesus of Nazareth, both directly and indirectly.
Moses is a bit more questionable, since there aren't many non christian sources pointing to him left. Some egyptian sources supposedly refer to a figure that could be Moses, but thats still open for interpretation.
On Abraham, he's just too far back and there's no non jewish/christian sources that mention him, I think, altough that is kinda expected from such an ancient figure. His existence is not unlikely, though.
As for the great flood, its doubtful it had the proportions described in the bible, but there is archeological evidence that a pretty big flood happened in the area in which the biblical Noah would have lived. Probably not a world destroying catastrophic event, but localized disaster? Not that unlikely.