The well-known Mormon historian Richard Bushman, in an interview with Bill Reel (~8:28), stated:
I think right now the Book of Mormon is a puzzle for us, even people who believe it hardily in every detail, it’s a puzzle.
To begin with we have the puzzle of translation: translating the book without the plates even in sight and wrapped up in a cloth on the table. So, it’s not something that comes right off the pages, the characters on the plates. So we don’t know how that works.
And then there is the fact that there is phrasing everywhere–long phrases that if you google them you will find them in 19th century writings. The theology of the Book of Mormon is very much 19th century theology, and it reads like a 19th century understanding of the Hebrew Bible as an Old Testament. That is, it has Christ in it the way Protestants saw Christ everywhere in the Old Testament. That’s why we now call it “Hebrew Bible” because the Jews never saw it quite that way. So, these are all problems we have to deal with. (emphasis added)
Bushman also recently stated:
The Book of Mormon has a lot of nineteenth-century Protestant material in it, both in terms of theology and of wording. I am looking for an explanation of how and why it is there. (emphasis added)