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.
Anti-Climax Boss: The last level in Clear Mode in the DS version, since the target is so huge (a giant box containing one of the player's friends). It requires 50 hits, but takes up most of the map and can be damaged anywhere. In context, most of the preceding levels have been packed to the rafters with indestructible gold bricks.
Demonic Spiders: In Arkanoid II: The Revenge of DoH, the black sphere enemies. Unlike most enemies, they would not be destroyed by either the ball or the shots from the Laser power-up - only being touched by the Vaus would destroy them. So not only would they deflect the ball, but they'd continue to be an obstacle until they moved low enough to be destroyed. In addition, they had a habit of "catching" the ball, which would not only shift the trajectory at random, but also the location. And they potentially can appear on any level, turning it into a Luck-Based Mission.
Ear Worm: Due to the way the tracks on the DS version were composed, potentially any of them could be this depending on music tastes, this being one example
Game Breaker: Depends on the level, so there's luck involved, but certain powerups can be this way. This is especially the case in the objective-based quests on the DS version - if the objective is to destroy a certain brick, or all of a certain color, and a Laser comes along then a level that takes a couple of minutes to win normally can be ended in ten seconds. Lasers and the megaball are almost always examples actually on any level that lacks gold bricks.
Arkanoid II: The Revenge of DoH shows why Mutually Exclusive Powerups are in effect - power-ups were divided into "affects the ball" and "affects the Vaus" effects, and one of each could be active, making levels much easier when Dispersion or Megaball was active at the same time as Catch or Laser. On top of that, one of the effects possible from the shining capsule was to create a double-strength Dispersion (16 balls instead of 8) that also were Megaballs. If that came up, a player would almost have to intentionally try to lose the level.