Given my opinion of the rest of the 'Callista Trilogy', and Barbara Hambly's Star Wars writing, the real question here might be, 'Why did you read this trash?' Maybe its because I'm a completionist, maybe its because I enjoy torturing myself...maybe, like Bantam, the Star Wars tie-in franchise of this era, and Hambly herself, I just can't stop throwing good money after bad. I'm going to try to tease out at least one or two positives here along with all the negative though. However, even the good has some serious flaws.

The Good(ish):
One thing Hambly does well here is the internal monologues of C3pO while he and R2D2 are on their own solo adventures throughout the novel. Though the repeated, and for that matter repetitive in nature, of their mishaps grew tiresome, it was enjoyable to follow them as separately from the rest of the cast of characters as we all know to a degree their perspective was key to the original trilogy ala Hidden Fortress. Getting C3PO's internal thought processes was a good way to flesh this out.
Hambly's writing style and language are also more elevated than what we typically get from a Star Wars novel. Unfortunately, that works to her disadvantage and probably should have been worked on more with her editor. The escapist Space Opera/Scifi/Action-Adventure/Space Fantasy nature of this franchise doesn't work terribly well with the more elevated language she tries to employ and feels shoe-horned and out of place and cumbersome rather than the more intellectual delight it could have been in other genres.
Finally, I know from browsing that Hambly writes a good deal of horror, and while I haven't read any of it I may give it a try as I think she might really shine there. Why, you ask? Well, some of the more horror leaning scenes in the book (primarily those involving Dzym and the drochs, but also Taselda) represent some of the most engaging and interesting writing here. Again, it just doesn't fit very well into the Star Wars 'genre'.

The Bad:
Whew...there's a lot. Some of the weaknesses and flaws in the Good(ish) above already touch on a few of the issues. A lot of the problems can be boiled down to this being an author clearly unfamiliar with universe/franchise in which she is writing. We have a lot of stuff about the Force and Jedi generally that makes absolutely no sense in the context of the broader Star Wars continuity, from them actively engaging with/using negative emotions as drivers for action to even more basic issues how Lightsabers work/behave ('tangling' together?), to Jedi that have simply had their powers degrade(?) over time somehow that goes unexplained yet mysteriously have their powers back when its convenient for the story. We have species represented in totally different ways from the established canon (Ho'Din being 25 meters tall?). You know, just an obvious lack of familiarity with the established universe in which she's working.
Rather than find ways to creatively incorporate Luke (and for that matter Leia's) powers as a Jedi into the ongoing story, we yet again have a lame plot device (the Force works different on this planet? but apparently only for Luke because the evil but somehow not darkside/sith jedi seem to have no problems here?) with scant information rendering him basically powerless.
Despite being a feminist author who frequently gives strong and interesting female characters in her non-franchise work, we similarly have Leia rendered effectively powerless, as well as without agency by being drugged a significant portion of the book. We also somehow have a (still)powerless (and uninteresting) Callista able to teach Leia more about lightsaber combat in one training session than in all the time she has spent with Luke?
As mentioned, we have Callista back, still powerless, and ultimately with no character evolution nor resolution. She's back for maybe the last quarter of the book, since, you know, this book ostensibly is still to a degree about Luke's bizarre search for her, and ultimately, not only do they never speak, they only see each other at a distance once, briefly, and she is (thankfully) never heard from again in the EU. Just as with the first two books of the trilogy, especially since we're well aware of the potential dark side/negative consequences of Jedi embracing love, we have a really out of place and nonsensical love story shoehorned in with no payoff at all. 
Speaking of the pointless return of characters...Admiral Daala suddenly re-appears in the final maybe 30 pages of the book. Even though up to that point we have zero reason to believe she would be involved. So, after her second resurrection/miraculous escape from certain, observed, death, she returns with still more Star Destroyers to...save the day? Given how little of the book she appears in, there is VERY thin rationalization for her change of heart, and certainly not accompanying character development one would expect to accompany it. Also, everyone just seems to be willing to forgive her for, you know, mass murder, planned genocide, war crimes, etc. and let her and her forces...settle on a planet to be farmers?
There's also a very brief re-appearance at the Noghri at the very beginning of the book. Which I would have been happy to see, especially as ostensibly part of the point of this book on Hambly's part was Leia coming to terms with her relationship with/to Vader. They could have played an ongoing and interesting part of that character development. However, within a page or two, despite everything we know about their prowess and skills, they are summarily killed and none of their kind show back up.
Its, frankly, a mess. It mostly feels like really lazy writing, combined with someone not really caring to learn anything about the franchise they're working in. The parts that *don't* feel lazy, feel like they were written for an entirely different genre or book.
Skip this, like the rest of the Callista Trilogy. A truly terrible waste of time, that I'm not even sure how it got published.