Unfortunately, Koch lost the balance in the subsequent books.
The characters should be a strength of the book since Koch has the creativity to come up with interesting character concepts, but she can't help just adding more secondary characters in each book. Then she can't let them go, so she doesn't really have time to do them all justice. Many scenes have a dozen underdeveloped characters to orchestrate, so half the time it ends up reading like Kitty (the sole POV) is an air traffic controller.
Worse, in a previous book, the author introduced a permanent deus ex machina in the form of ACE - a superconsciousness that has pretty much unlimited knowledge and power and helps them out at random. There's some hand-waving about free will explaining why the characters ever have to anything for themselves, but basically it's the author's get-out-of-jail-free card since ACE loves Kitty. Grrr.
Speaking of loving Kitty, another thing that is sinking the series for me is the number of guys that love Kitty. All the (many) males in her orbit either either currently love her, formerly loved her, love her in a platonic way because they're gay, love her in mostly platonic way because of the age difference, or are clearly evil villains up to no good. Ugh. Too many of her characters are already underdeveloped and making all the men gorgeous, intelligent, well-hung action heroes who love Kitty makes the problem even worse. Not that Kitty cheats on her new husband. Koch may have a fallen into a number of traps, but she's avoiding the full Anita Blake-style descent into male harems.
A pitfall she is not avoiding is the escalating powers problem. She's trying to have each book top the last book, and so she's escalating the power level. It's a common problem in fantasy books and RPG games. If the main character(s) keep getting more powerful, then how to you keep them challenged? If you just make the other side more powerful in the same way, then after a while it gets tedious because all the souped up powers end up just using more cognitive space and action time without changing the nature of the challenge. Usually an author or GM will change the types as well as the scale of the problems the character face so that the challenges feel different as the power level rises. It looks like Koch is planning to do that in her next book, where the main characters *SPOILER* settle into the diplomatic corps *END SPOILER* , but she should have done that a book or two ago, I think. We'll see if it helps or if it's too little, too late.
Since this is turning into a rant, I have to say another thing that bothers me is how she handles the key new addition - Kitty and Jeff's new child. I've had three kids, so I speak from experience when I say that you fall in love with your kids in an emotionally complex and physically-yearning-to-touch way that is just as powerful as a romance. Although Kitty tells us that they love her and they take a couple minutes to hold her and kiss her, mostly the baby is treated as just something to work around (with much talk about Kitty's "torpedoes" in reference to needing to breast feeding) and as a new-powers plot device. This was an opportunity to have an emotional arc in Kitty world as powerful as the relationship between her and Jeff in the first book, and Koch glossed it over. Argh. The pain of missed opportunity.
Other books in the Alien series: