The Adversary
The final volume of The Saga of Pliocene Exile delivers an overstuffed conclusion in which murderous abusers edge toward redemption, ever-expanding psychic powers dissolve the stakes, and crucial information is withheld simply to keep the reader turning pages. Includes a complete spoiler-filled recap for anyone who'd rather skip the 470-page ordeal.
The Armies of Daylight
I stuck with Barbara Hambly's Darwath trilogy out of love for the author, only to find an ending that made three books of struggle feel pointless. Flat protagonists, frustrating plotting, and one of the most unsatisfying finales I've ever read.