"Elsewhere, the dialogue often sounds a little too modern ,,,"

Oh, no...not this again!  Get it enough on the small screen, with Vikings saying things like, "Let's get it done" and 16th century pirates espousing obvious 21st century social issues--in 21st century terminology.  

Yeah, I know, connecting with the audience, etc., etc.  

Plain speak is better speak, and connection would be instantaneous.