They just don't make films on this scale anymore. Even the epic series "Rome" depended on clever use of CGI to 'multiply' mere scores of people into multitudes. And, in the late 40's and early 50's, the surrounding countryside still looked sufficiently untamed to convince one that the legions tramping down the Appian Way really were in the world of 2000 years ago.
I suppose the sticking point, for me, was Nero not only ordering Rome to be burned, but actually playing the lyre while it burned. The truth is that there is no evidence he had anything to do with the fire, and had in fact raced back to the city to try stopping the blaze from destroying the entire city (probably the one decent thing he'd done in his entire life.) Nevertheless, the story was entertaining, and it was quite fun watching Ustinov (in his younger days!) playing the Mad Emperor. Definitely worth watching, just for the lavishness of the costumes and sets, and the sheer spectacle.