While the cinematography is out for everyone to appreciate, I feel a need to stress a couple of key points international audiences might miss.
First and foremost: the series is based on the first of four books about the life of Mussolini, and while they are routinely described as "novels", they are in fact impressively researched from a historical point. Pretty much every dialogue, speech, document and event are openly sourced and beyond question.
While the staging frequently and blatantly is not realistic in order to make the show more spectacular, pretty much everything happened as you see it - including a D'Annunzio's Japanese samurai secretary and other apparently absurd elements.
My second point is about the acting and language. While it will fly above the heads of anyone not _very_ experienced with Italian, the work behind regional accents, lapses into dialects and the very speech patterns of 100 years ago is exceptional indeed. It is a shame this will mostly go unnoticed, as it actually conveys a fair bit of refinement to an already stellar ensemble performance.
Lastly, the (not present in the books) stabs at current politics are far more varied than expected. Like, everyone can understand a sudden fourth-wall-breaking "Make Italy Great Again!", but you'll probably miss the reference to the catastrophic Berlusconi era (delivered via subtle voice and body acting, or even just a brief costume change), or to the sorry state of the last few recent Italian legislatures.
In short: come for the show, stay for the history lesson... and see it with an Italian resident if you can to enjoy it at its fullest.