After reading the Probability series, I have come to the conclusion that Nancy Kress' greatest strength is getting the reader to turn the page. This is an important skill, because without it no one will read your books, but it doesn't necessarily make your books good. And Kress' Probability books have some major holes.
The biggest hole is character. Kress seems to feel the need to include an extremely annoying character in each book. Some people are annoying, and it stands to reason that some characters will be annoying as well, but if that character makes the reader want to put the book down, the author is doing something wrong. Character inconsistency is another problem, especially in Space, where Kaufman, previously my favorite, acts completely out of character and becomes a real ass. But the coupe de grace is that the big finale--the destruction of space-time as we know it--depends on two completely unbelievable characters: Admiral Pierce and the Fallers. One, supposedly intelligent enough to gain control of the entire empire, is too stupid to believe in science; the others are so spiteful they would rather destroy space-time than see one of their systems in control of the enemy, despite the fact that they were previously so desperate to preserve space-time that they handed over important military secrets.
Perhaps the unbelievability of the Fallers should fall under the heading of anthropology, which is Kress' other major hole. Humans know an unusually large amount about Faller psychology, despite the fact that they've never (!) intercepted a single Faller transmission and, prior to Sun never had any kind of meaningful interaction with them. But the issue of the Fallers pales in comparison to the issue of World and the creators of the artifacts.
Most of Moon is taken up by interactions with Worlders, as is much of Sun, yet in Space they are almost a no-show. Considering how much ink was dedicated to them in the first two books, the reader might assume that they are actually important to the plot. Not so. In fact, World is only important because it contains the two artifacts; if you strip away all the interactions with the natives (which are now revealed to be superfluous), you could probably fit the entire trilogy into a single book. Why include so much of World, including a very major POV character, when the people of World really don't matter? It will remain a mystery--along with the creation of the artifacts that are the driving force behind the trilogy.
The two artifacts found on World, as well as the space tunnels, were created by a mysterious and long-gone race. The constructions are absolutely key to the books; without them, there would be no story. Yet almost nothing is told about the people who made them. In Moon the reader learns there is some rudimentary knowledge of their language, apparently gleaned from the space tunnels alone. There are apparently no other relics of these people, no archaeological sites, and no theories about what happened to them. Worst of all, no one seems to be at all curious about them. Throughout the entire trilogy, people use the space tunnels and examine the artifacts without sparing a second thought to the race that built them. Apparently, in Kress' universe, humans have evolved past curiosity, which perhaps explains why no one would bother to wonder why a planet like World, which rates only a single space tunnel, had two enormously powerful artifacts that no one had ever seen the likes of before.
The Probability series is interesting. It keeps the pages turning. It is not life-changing, but it passes the time. But by the end, the holes have become so large they begin to interfere with the suspension of disbelief, which is a dealbreaker in sci-fi or fantasy book. I would not dissuade someone from reading this trilogy, but if someone asked for my opinion the best I could say would be "It's OK."