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.
The Walls of Air
Rudy and Ingold undertake a perilous journey while Gil and Minalde investigate an ancient keep. Intriguing mysteries and stronger characterization can't quite overcome frustrating plot contrivances, questionable decisions, and a truly baffling approach to food storage.
The Time of the Dark
A medieval historian, a drifter, and a magnificent old wizard find themselves fighting to survive an apocalypse in another world. An unexpectedly thin plot and unconvincingly resilient protagonists disappoint, but Ingold nearly saves the whole thing.