My only major problem with the historicity of it are the constant threats by the various parties to have someone crucified.Crucifixion was not permitted as a sentence for Roman citizens. It is why St. Paul was beheaded while Jesus and St. Peter were crucified. Aside from that. I'm really enjoying it, although I wish there were more scenes of the Roman Army in combat. Having lived through third year Latin, I can't wait for Cicero to get his.
You touched on my least favorite part. Given all the bragging about the huge budget for this series, the montage treatment of the battle between Ceasar and Magnus felt really cheap. It totally reminded you that you were watching a TV show and not an epic film like Spartacus or Gladiator.
Crucifixion was the worst form of several types of capital punishment, and , although seldom employed on Roman citizens, it was within the law, as the defense by Cicero of Rabirius ( a Roman citizen and in fact a senator ) against the penalty of crucifixion in 63 BC demonstrates.
There are instances of Rome employing crucifixion on its citizens, such as in 66AD by procurator Gessius Florus.
In general, from its earliest days, crucifixion was almost exclusively used on slaves and noncitizens.
As a matter of curiosity, one of the 2 judges who found Rabirius guilty and occasioned Cicero's appeal happened to be none other than C. Julius Caesar; the other judge was Caesar's cousin Lucius, keeping the verdict in the family. Of course it is absolutely irrelevant that both were champions of the "popular" party, while Cicero and Rabirius adhered to the other "senatorial" party. You could think of this as something like the 9th Circuit handling the DeLay matter.