The Inferno features several elements based on ClassicalMythology, including Cerberus, the River Styx, and Plutus. Once they're at Purgatory, though, it's pretty much all Christianity-based lore. Well, since hell is where all the sinners go, then it would naturally serve as a receptacle for the blasphemous ideas of a "false religion".
However, in Purgatory the typical structure of the examples of virtues and vices is to have a mix of examples from the Bible and ones from classical mythology or Roman history.
If Ugolino is gnawing on Ruggieri's head forever, then what does that mean for Ruggieri? Will Ugolino only eat so much of his head so that he ends up being stuck with a massive hole in his skull and/or at least part of his brain missing? Or will Ugolino eventually eat his entire head, leaving him headless? And when Ugolino is done eating Ruggieri's head, what will he do next? What happens when both of them get their bodies back at the Final Judgment? Will Ruggieri's head be restored, only for Ugolino to eventually eat it again? Or will his head remain the way it already was?
When Dante passes through the fire on Mount Purgatory, Virgil isn't there and he cries. Virgil is never seen again after this. Although Virgil usually pretends otherwise, he is constantly sad due to the fact that he is forced to spend eternity in Limbo as he cannot go to heaven because he died a pagan. Virgil even mentions this explicitly in Purgatorio Canto III. While God permitted him to leave Limbo to guide Dante through hell and purgatory for a few days, he is eventually forced to return to Limbo once his job is done. Dante finally reaches heaven in the third cantica, but this means he will spend eternity separated from his guide who he only met for a few days. Meanwhile, Virgil will spend all of eternity grieving God's presence just like all of the other souls in Limbo. Although Virgil helped Dante reach heaven, he can never go to heaven himself.
