I could live with some of the errors in lore, timeline and background. Though, frankly, their sheer number is completely inexcusable, since all it would take to avoid most of them is to actually have read the previous books or played the games - something the author quite obviously couldn't have been bothered with. Let alone research some of his stuff on one of several mass effekt wiki pages...
But what's really appaling is the way this book cuts off the trilogy, with, basically, a complete lack of a story.
***spoilers ahead***
The book is advertised as a bridge from Mass Effect 2 to Mass Effect 3, which couldn't be less true. The only thing Deception does is to "clean house" and completely sever all loose end left from Ascension and Retribution by killing off all characters introduced in these books - and not needed or prone to appear in ME 3 - in the most pointless way possible. There is really no new story told here, no new elements or characters introduced, the events are unspectacular, unrelated to the bigger picture, and just lead nowhere.
What's worse, Deception basically spoils Ascension and Retribution, as all Events in those 2 books are rendered entirely meaningless. Gillian for instance, built as a character over 2 books, is killed off in passing, as are Hendel and Nick. The other characters, who are likely going to make an appearance in the 3rd game, are carefully left untouched, and thus, remain utterly bland.
Worst thing with a BioWare logo on it since Dragon Age 2...