These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Complete Monster: Pryrates is an Evil Sorcerer whose only established motivation is the thirst for knowledge — all knowledge, no matter how dark or forbidden. He first shows up as a junior member of the League of the Scroll, a group devoted to preserving ancient knowledge against the possible return of the Storm King. When Cadrach, a fellow member, unearths du Svardenvyrd, a forbidden book containing a prophecy of the Storm King's return, Pryrates tortures and Mind Rapes him for its secrets. He then ensnares King Elias in a web of deception, posing as a trusted adviser who can help him communicate with the spirit of his dead wife, but instead leading him into selling his soul to the Storm King for the promise of immortality. He provokes Elias to war with any who threaten this goal, including his own brother, whom he captures and attempts to sacrifice. To further his plot, and out of personal pride, he personally murders the Lector of the church of Usires Aedon (equivalent to the Pope). His quest for power is not stopped until he finds out that betraying the Storm King is not a good idea. And if this weren't enough, his Establishing Character Moment is crushing a puppy to death beneath his boot while the hero is watching, just because he can.
Goddamned Bats: Bukken, tiny diggers who like to burrow beneath the ground and swarm larger foes from below. They have a tendency to show up in the most inconvenient of places.
Growing the Beard: The pace of the story picks up noticeably about halfway through the first book and goes to warp speed by the end of the third.
Moral Event Horizon: Averted. Pryrates, being a Card-Carrying Villain from his first appearance, starts out beyond the M.E.H., but Williams goes to great pains to make sure the story's other antagonists, including the Big Bad himself, all remain to some degree sympathetic no matter how many awful things they do.
Nightmare Fuel: The ghant nest... shudder. The tunnel sequences with Simon can also be brutal for anyone who suffers from claustrophobia.
Trapped by Mountain Lions: The entire extended subplot involving Miriamele, Isgrimnur, and Tiamak finding their way to the Stone of Farewell might qualify as this.
The key word there is "might"; their seemingly pointless wandering provides crucial world-building and adds depth to the main plot by putting viewpoint characters at key scenes/locations.
The Woobie: Examples of tragic suffering abound on all sides of the story. On the villains' side, there's Elias himself, who is tricked into the whole Evil Plan out of grief over his dead wife; and poor Guthwulf — as much of a Jerk Ass as he is to begin with, he doesn't deserve to be Mind Raped by Sorrow and blinded by Pryrates. Nearly every plot-significant protagonist suffers, too. Maegwyn and Leleth in particular seem to have been screwed over by a cruel deity just so they can sacrifice their lives to give Simon the strength to return to his body from near death.