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.
His V3 is (or was, at least) bugged; his forms are supposed to have separate HP totals, but they don't. One effect of which is that once you inflict enough damage to any elemental form to get him to discard it, he'll already be low enough to discard all the others as well and so won't attack you. This makes it very easy to kill him before he hits his final form (and that form still only has 200,000 HP, which by then your boss-killers can easily do far more than per hit, so even if he does reach it, he's still screwed).
Breather Boss: Pretty much every boss on the 19th floor, given what you had to go through on the 18th.
Game Breaker: Most attacks with a high damage multiplier and/or that can ignore the enemies' defense, such as Nitori's Megawatt Linear Gun or Eiki's Last Judgement. Some attacks, however, may seem like this, but are balanced out by some weaknesses (Rumia, for instance, has both kinds of attacks, but her average magic stat does her no good).
Nitori again in the second game. One of her special skill is called Maintenance which doubles the bonus of equipments. This includes bonus from ailment resistance, and elemental affinity, solidifying her defenses to tank-like levels and making her hit like a truck, that is on top of having the second high-multiplier attack from the first game.
The subclass Diva allows the character to constantly push back the turns for every enemies and all allies except the caster back. Repeated usage will cause everyone else to have their turns shifted to 0. While this may sound useless, if you have someone that is fast enough to outpace the enemies, they will get their turn before the enemies and before the Diva pushes back everyone's turn again. Done right, you can deny the enemies to make a single move for the entire battle.
Goddamn Bats: Any random enemy with absurdly high speed, due to how the game punishes you for finishing a fight with less than full health.
That One Attack: Djinn Storm, which depleted your entire party's SP, including the back row.
Great Ruin from the second game is just like Great Calamity/Djinn Storm, except it also reduces your frontliners' HP to 1.
That One Level: Floors 10-12 are a nightmarish 3-floor lock puzzle with an insane amount of backtracking. Floor 7 is also notable for being a frustrating teleporter maze.
That One Sidequest: Recruiting Suika in the second game is easy enough, but until patch 1.140 it required you to collect a total sum of 216k money and spend it on increasingly expensive items in Nitori's shop. To put that into perspective, there's an achievement for getting 10k money in one go, and you're not likely to get that until rather late into the game.
Actually, disabling it was the fix. EVA did work in the initial version of the game, but given how stat growths work (and the fact that since there's no accuracy stat, accuracy doesn't increase to offset this), it ended up being completely broken. Rather than changing the system to make it works (which would have required quite significant changes), it was simply disabled. The sequel does have working EVA, integrated into the system in a non-broken manner.