The visuals had a problem, too (omitting for the moment Trumbo's hatchet job on the person and life of Marcus Licinius Crassus).
There was a broadly-applied anachronism in the arms and equipment of the Roman soldiery, which was from the Trajanic period 175 years later.
Roman soldiers of the first century B.C. wore simple Montefortino helmets (whose Roman name was conus, so you get the idea) and chain-mail, thigh-length hauberks, not the laminated plate armor seen in the film. The shield form was also wrong, and the swords used in Caesar's and Pompey's time was not yet the straight-edged, simplified form called "Pompeiian" (one was found there).
Just gratuitously complaining about the details; but the larger complaint I've read and am unable to criticize is that the real story of the Servile War, the revolt's actual name, is contaminated by "Spartakist" German Communist pseudo-historical "lore" which advances the Marxist-Leninist historical narrative.
But what people like about the film is Kubrick flexing his directorial muscles with panache, not the bloated melodrama of the script.