I think the negative feedback on part is due to the genuine understanding or concern over the 'ills' of our current society. Perhaps the resemblance to the fall of the Roman Empire is not appreciated as much.
Ambitious to pack multiple ideas into a single movie, most likely contribute to the confusion and perceived incoherence.
Portrayal of characters might come across as confusing sometimes - do some characters/actors really know what they are playing? I don't quite see Cesar being very well played as an ambitious ideologist while being a bureaucratic on the façade, or Julia being an intelligent and rebellious 'heir' who should portray the debauchery that the 'class' of hers so used to, and perhaps the little voice of longing for something deeper and meaningful that was supposed to flourish more as her time with Cesar prolongs.
Just perhaps to bring the spirit of ancient Rome to the modern era is too formidable of a task.