My big issue with onscreen Alternate History is that a screen presence demands a narrative- something you don't often get with AH. Once you have narrative, you then need a story to tell if you're telling a more conventional story (if you aren't then accusations of making propoganda, or worse, documentaries, start flying) and then you start running into the devil of genre conventions. At this point, the alternate history has dropped from the focal point to being a setting, and from there who cares when a few corners get shaved? It's a setting, and building a setting is damn expensive when you're buying props and building sets. Try and bring the alternate history back into focus, though, and you can start losing the character that really draws the audiences in.