In particular, the aliens' evil plan is uncomfortably close to the Holocaust. As has been discussed at length by people better qualified than me, this is one of those topics that Who can never really do. Doctor Who, however seriously we may sometimes take it, is fundamentally entertainment; the Holocaust is not. The book has a gross error of taste and judgement at its core.
(I have enjoyed Bishop's later work - particularly the second series of Sarah Jane Smith audios - but this is a wobbly start to say the least.)