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.
From the game:
Game Breaker: Some of the storyline and secret cards have ridiculously useful abilities, like "increase all allies HP max".
Cait Sith is a good example. Immune to spell cards and can't be damaged by creatures with more than a certain base attack. Set up on a max level fire space, only a few card can even stand a chance of killing it, though a scroll attack (which penetrates the defensive land bonus) will kill it in one hit.