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.
Badass Decay: This game saw the debut of the Immortal cards, which serve as a set of Infinity Plus One Swords. They had powerful effects, even aside from the natural benefits of being Immortal (immune to Crush terrain, and create it upon death, likely killing any opponent who beats them). Unfortunately, when they were released in the actual game, they mostly lost their awesome effects or were just plain not very good - only Slate Warrior and maybe Rigorous Reaver were still sorta playable. How bad was it? One of the Immortals was Shapesnatch. Yes, that Shapesnatch.
Critical Dissonance: The game got mixed reviews, but it still became a Greatest Hits title with over a million copies sold.
Critic-Proof: Likely the reason for the above. It was the first Yu-Gi-Oh! video game for the Playstation 2, released in 2001 at the height of the franchise's popularity — it was going to sell no matter what critics said.
Designated Villain: The Yorks are supposed to be the bad guys, but if you play both campaigns, it's pretty clear who's offering the better deal. If you side with the Lancasters, Henry VII gets crowned King and takes the credit for winning the war, to the point where you, who did all the work, aren't even mentioned in any recorded history, and it's unclear if you even returned to your own time. Meanwhile, siding with the Yorks leads to Seto treating you as an equal, even planning to share the the Guardian's blessing with you. When the summoning doesn't work out, he gives you a pendant that guarantees his family's debt to you, and gladly returns you to your own time. Furthermore, we're told that the Yorks have the Lancasters pushed into a corner, but a quick look at the actual map shows the Lancasters being everywhere and equally strong as, if not stronger than, the Yorks. On top of that, regardless of what side you choose, Henry becomes king anyway and Seto steps aside.
Game Breaker: A whole lot, the worst of them being very difficult to obtain, but the biggest one has to be Darkness Approaches. It flips everything on the field face-down, meaning flip effects can be recycled. A good number of cards power up themselves when they're flipped face-up, such as Wood Remains and Mystical Elf. Not only that, Fiend Reflection #2 lets you play another card the turn it's flipped, and even worse, Magician of Faith and Mask of Darkness can recycle spell cards. Including Darkness Approaches, which then flips them face-down, creating an endless cycle. Admittedly this is a card you need to be very lucky to obtain.