Too many storylines!
This book follows roughly 6 story arcs at one time that are happening on 3 different planes, and maybe 6 continents over the course of 5 centuries. Some characters are immortal, some are semi-immortal, others mortal, and may even die off-screen after being followed somewhat by the storyline.
Given this tangled web, characters like Karn or Rofellos can disappear for extended periods then have a shockingly unceremonious reintroduction to the story. All the while you expect these inductive cords to come together in a profound way, but many just quietly and individually resolve. The complexity is the killer for this book. Agreed, the ending is not compelling, but at least you know what's going on at that point.
Like some other finales, you I kind of just finished this to know how it all turns out.