In a way, I admire what Turtledove's trying to do - Great War, American Empire, and the next fourteen sequel trilogies will be definitive and exhaustingly detailed. The problem I have is that he concentrates too much on North America. That may work fine for a single book or even one trilogy, but if you're writing a complete alternate century (as he seems set to do), you have to examine cause and effect in other places in more detail than just offhand comments from American characters about an event occurring in Germany or wherever. Rather than 5 billion North American characters, how about a balance?
It's as if the series is meant to convey the typical American reaction to living through these tumultuous events. Again, that's fine for a book or two, but it becomes verrrrrry repetitive through three or four trilogies. We need a broader perspective!