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.
Anticlimax Boss: The Ura leader is extremely powerful and hard to kill... in the Stranger's Dream, where you're stuck with normal weapons and weathered 30 waves to get to him, plus he's got Queen Anne and a poison archer for backup. In the game proper, he's a pushover because you've got the insta-kill Battering Ram which murders him in one shot. Even in the Stranger's Dream, though, once you take down the archer, the strategy to kill him is as simple as "block, attack, roll back to keep Anne at a distance, repeat."
Lunkheads, leaping enemies with an armored face that do extreme damage if they hit you. The strategy to beat them is simple enough, getting them to obey a consistent leaping pattern to implement it is not. They're much easier once you get armor-piercing weapons, but a nightmare otherwise.
Anklegators, essentially moving spikes that pop up if you walk over them. They're easy enough to dodge at first, but their speed and reaction time increases with damage, and they send you flying if they connect.
Stinkeyes, flying bombs which are weak but actively shield themselves from ranged attacks except those that can be fired instantly. While their eyes are closed, only a weapon with a poison/bleed effect can do any real harm. Worse still is the giant sack enemy that spawns them and takes a lot of effort to kill.
The hammer-spinning Gasbags. They're tougher than all but the biggest gasbags, have a huge hit radius because they spin, and you can't counter them like other enemies, just block them. They also give you very little time to use melee after they stop spinning.
Large Rattletails. Ranged attacks don't work half the time because they burrow underground to chuck rocks at you, they move around constantly when they are above ground to make melee much more difficult than it should be, and they almost always come in packs. The little ones, at least, die in a couple hits.
Ura warriors also qualify, since they teleport and move extremely fast. Not only that, they teleport when hit some of the time, which can actually allow them to hit you in the back with no warning. Finally, certain variants get a charge bonus, similar to the Kid when he's charging an attack, except this boosts their defense and attack speed with no downside.
Special mention has to go to the poison archers, who as the name suggests shoot poison arrows which drain health and inflict an Interface Screw. They also get the charge bonus, making them even harder to kill. For added annoyance, the Stranger's Dream sends four at once to kill you, plus two more down the line, the last being in the final level while fighting two boss characters.
Everything becomes this with the idols on. Even the Goddamn Bats become murder machines if they can land a hit.
8.8: Jim Sterling's infamous review, which gave it a 6.5, a stark contrast with the game's otherwise positive reviews.
Ensemble Darkhorse: Rucks. Ask anyone who hasn't played the game but at least knows about it what they know about the game, and they'll probably tell you about the gravelly-voiced Lemony Narrator.
Game Breaker: Werewhiskey, which gives 100% critical hit chance at 33% health. Combine this with Lifewine and Bull Brandy to maximize your survivability and it doesn't matter that you're constantly at critical health, because almost nothing in the game will live long enough for it to matter. This is especially true in the weapon challenges, which become much easier simply by falling off the edge enough to activate Werewhiskey's effect.
Goddamn Bats: Squirts and Peckers are swarming weaklings that usually act as distractions for bigger enemies to clobber you. Miniature Rattletails also count.
Good Bad Bugs: When you arrive after either saving or leaving Zulf, if you decide to do the Stranger's Dream, you will be able to jump and use the Battering Ram to fight.
Rucks: Kid wakes up... nah, I'm just foolin'. He sees what's left of the Rippling Walls, years of work undone in an instant. He sees what's left of Pyth, the bull. The gods...they're all undone. He sees what's left of his lifelong friend. His friend...he's come undone, too. He sees. What's left. Undone. Undone.
If you fall off into the abyss during this sequence: "Kid can't fall no matter how hard he tries."
That One Level: Point Lemaign. Narrow platforms with Lunkheads at every chokepoint, Stinkeyes on the elevator portions, elevator portions that swarm you with enemies (especially the second one, which is flat-out ridiculous), all coming together to make a level that will have you pulling your hair out. And if you came in using idols? Yeah, just kill yourself, because they'll surely do it for you.
That One Sidequest: The Proving Grounds vary widely in terms of difficulty. Some, like the Galleon Mortar, you can conceivably pass on your first try. Others are only tricky (Breaker's Bow) or nearly impossible (Fang Repeater) until you get the appropriate upgrades or learn the right trick (War Machete). Then there's the Bullhead Shield trial. There's no upgrade that will help significantly, no trick to make it easier. Just your timing and luck. You'll need plenty of both.