Finished it today. Don't get me wrong in what I said previously. It's a good read. It strains credulity at certain points but damn me if it doesn't hold your interest. I've already purchased the next two in the series.
That said, S.M. Stirling is basically a 14 year old boy. He likes swordplay and armor and catapults and cannibals and witches. He clearly wants to write about this sort of thing but he wants to be able to do it using modern characters. There's nothing wrong with that.
You need to understand what this is though. This isn't what I'd call a thought-experiment. This is fantasy literature, pure and simple.