I just want to get out of the way that your review seemed to lean on the side of African American slaves as being stereotyped. Look, that's how they were in the seventeen hundreds; they were slaves that were forced to learn English, and they're completely different from Africans of today, or for that matter, within a hundred-year radius from us.
Now that that's out of the way, here is my review. I thought that Johnny Tremain was a fairly enjoyable story. The good parts about it were how the characters developed, hands on witness on important historical events, and exploring the eighteenth century world as portrayed by Esther Forbes, who does it perfectly in my opinion.
The downside, however, is like you said: it's dry. Of course, there are aspects that make the book interesting, such as the possibility of Johnny being related to the wealthy Lyte family, and the meetings and plans of the Sons of Liberty. Unfortunately, about a whole third of the book is further EXPLAINING what is happening and throwing in pointless idioms and phrases, further confusing the reader about what is really going on. Plus, the language at the time, though not as different as, say, works by William Shakespeare, can be quite confusing, as I said earlier.
Fortunately, this book is overall a decent read and I'd recommend someone at least check it out.