Its main strength is scope. Wisely the authors do not try and bog down the absurdity of the plot in one place and time. Instead they opt to jump across time streams and location never quite giving the reader the opportunity to get bored. Whilst this means that one never quite gets a sense of belonging to any part of the book it does ensure that one feels like a casual observer to something worth turning the page for. If one were to dissect the plot of Shadow then one would probably think it quite dull.
Spaceship is shot down, aliens treated badly, aliens want to get home.
Tie this in with the machinations of a rather vapid Fourth Reich led by 'is he or isn't he the real Hitler' and one more or less gets the point. The danger with broad scope is that one misses the opportunity to go underneath character and place. However, given the plot of Shadow this is not a missed opportunity at all rather a blessing. Given that one of the guidelines for BBC Books submissions is that the Doctor should be central to the plot from the word go it is odd that almost a tenth of the book has him absent. His size provides the momentum without having to develop too much of one elsewhere.