For a lot of people, Christopher Bulis has come to be synonymous with a certain style of Doctor Who writing. Sometimes it was called "trad", others called it "safe"...the more pejorative terms might have included "bland" and "hacky". But the basic concept behind all the definitions was the same. Find someone who can produce X number of words in Y number of days, with a plot that holds together reasonably well and a prose style that doesn't actively grate on the reader. If they can come up with a few witty lines of prose here and there, that's a benefit, but the main thing is that they should come up with Doctor Who that feels a lot like the Doctor Who people have already written and already read.
And yet, 'Shadowmind' isn't quite yet the work of the man who wrote 'The Eye of the Giant' and 'The Ultimate Treasure'. His regular characters don't feel on-model (the Doctor, for one, feels so much like a generic Wise Mentor figure that Bulis has him recite an actual list of old series monsters he fought, just to establish his credentials) and his prose is staid and straightforward, but his characters are likable (Kim Talevera and her grandfather are sweet and gentle people) and his setting exudes a certain fragile charm. The Shenn, for all that they're pretty much just Stock Sci-Fi Alien Race #22, are cute and fluffy and friendly, and Umbra, for all that he's Stock Sci-Fi Monster #27, at least has some interesting science behind him and exudes a sense of genuine menace in that way that evil, powerful children can.
Which is why the ending is all the more strikingly off-tone. We may never know whether Bulis was forced to end the book with a bloodbath by his editors (who were, let's face it, keen to develop a "mature" sensibility) or whether he felt that the book would seem more "grown up" if he killed off half the cast at the end, but it never feels like it fits with the rest of the book. The first three-quarters of the novel feel like he's developing a good Doctor Who novel for children; the last quarter feels like he's pulling the rug out from under those children instead of writing a book for adults.
That's not to say that there's nothing to recommend the book; again, the time spent on Tairngire feels calming and restful, and the crew of the Broadsword is likable and intelligent in a way that's just nice to read about. (John Rogers once called this "competence porn".) Even the plot, while not innovative, is a pleasant way to waste a few hours. It's hard to feel good about disliking Christopher Bulis, because there's not even anything really to hate. You just sort of want to give him a B- and explain to him that you'd like to see him try a little harder next time.