The historical accuracy was not a factor for me because I hardly know anything of it
I really didn't see the germans as incompetent, but now that you mention it, the book might have leaned over towards stupid germans and smart russians. I do beleive it was more or less the other way around. I don't know why, but I found the story really good in the book, and I liked the way it was written.
When Nikki led his troops around in Stalingrad, he seemed, as did the other germans following him, pretty smart and competent. Anyway, that's just my view on it, but I don't know very much