I believe they only have the rights to whatever is in the appendix of The Return of the King, so they don't really have a story to go off of. But it's clear at this point that they are basically rebooting the history of the second age. The names and characters are the same, but the history is totally different. And I'm ok with that, really, since it's not like they are re-writing The Lord of the Rings or The Hobbit.
My issues with the show are mostly about how tedious most of the dialogue is. I don't know, maybe it's just that the story of middle earth is more interesting in the abstract, or on a macro level, then on a micro level.