Most miscast lead role ever?

The main market for this series is Europe, where it's dubbed into Italian, French, German, etc., or it's shown with subtitles. Non-native English speakers, even if they watch the show in the original English, aren't likely to notice Doman's American accent. Personally, I watch this series in Italian, although I generally try to avoid dubbing, because characters speaking English in a Renaissance Italian setting sound just unnatural to my Italian ears. The characters in the Italian dubbing have no regional accent (which isn't historically accurate), with the exception of Giulia Farnese, who has a very light upper class Roman accent.

Just watched Doman playing a modern-day bishop in season 3 of House of Cards. He's actually very good in that particular role (the character is a spiritual man who we're told has a fondness for motorcycles, and who doesn't mince words - almost seems to almost have a bit of NYC cop in him).

So all we not-english/american people have one great advantage, we don't hear him with an american accent, his dubbing voice is good, and we can enjoy his acting, and in this, he's doing fine, as the rest of the crew.
I understand that for you as an Englishman it may be annoying to listen to someone with an american accent in a british production, but for the rest of the world, this doesn't matter at all.

English is not my first language. Any corrections are welcome.