They are trying to do something very different for each book. I didn't expect this one to be set so very far away from the core solar system, with no opportunities for assistance within months of travel time, and (quite frankly) monsters coming at our fearless protagonists from all sides.
I'd say this one is the biggest nail-biter so far (and I've already read the next one, spoiler alert). Not to give too much away, but important stuff doing a slow slide into the upper atmosphere while everyone scrambles around trying to keep that from happening... That's pretty darn thrilling. Not to discount all the exciting things happening on the ground (even more so, at least towards the end).
It's clear that The Coreys have thought long and hard about the arc of these 9 novels, so that you get a different aspect of space opera each time, with each one leading to the next seamlessly. The only thing that disturbed me in this one was the super-long underground subway system (shades of A World Out of Time, anyone?) that seemed to work so darn... seamlessly after millennia.