The writers and makers of TV shows are frequently a lot more fallible than we'd like to believe. They get caught up in what seems to them like a really good idea and lose perspective, which leads to deep frustration among viewers because they get some bits so very right, and other bits so very wrong.

Indeed!

I also think that series one was the most well-crafted, but I have enough of a preference for Billie and David's chemistry that I'd probably pick two over one if forced to choose.