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.
YMMV: Digimon Digital Card Battle
Breather Boss: Everyone at Junk City is considerably easier than the preceding boss, Garurumon. It's possible that they're even easier than the opponents in Beginner City.
Game Breaker: Download Digivolve, Rosemon's Lure Card, Hacking and the 7-S cards in general.
Some support effects borders on this, especially those from Rare. The most prominent example however, is Toy-Agumon's support, by discarding one random card from your hand, you reduce the HP of both sides into 200. Yes, including Ultimates. This turns the Big Bad that uses The Computer Is a Cheating Bastard to the max into a total joke.
Yellow Banana deserves a mention too. For half of your HP, you counter any attacks. In practice, it mean that if it's not countered, you will survive the next turn on top of possibly dealing damage to the opponent.
One of Gatomon armor digivolve, Tylomon have a X attack which have the special effect of doing triple damage to red opponents. What make it game breaking is that unlike other partners, you don't have to equip a part that will lowers your X attack to have such an effect on Tylomon. Add a part that boost your X attack and the damage against red Digimon will skyrocket. Just watch out for counterattacks.
Good Bad Bugs: According to its description, the Special Digivolve card allows you to digivolve your Digimon into one of a different specialty, adding 20 DP to the amount normally required for digivolution. What it actually does, however, is reduce the required DP by 20. Additionally, Special Digivolving a Champion which was not digivolved from a Rookie (making its stats halved) will result in an Ultimate with its proper stats.
Junk Rare: The Fake Sevens and the 12 Digi-Jewels cards are only obtainable via fusion mutation, an event that only has a 1% of chance of happening while fusing cards. Trying to get all of them requires a lot of time, patience and spare cards from the player and their effects are so low better results could be achieved by using some of the most common cards in the game. Fake Sevens is easily the worst of them, as it does absolutely nothing.
Nightmare Fuel: The ending gets really dark. After defeating Venom Myotismon he starts repeating the same lines over and over. Then the game starts being hacked by a man known as A which causes the screen to go black and later all the colors change. At this part of the game, the player regains control but can neither live the town or save progress making this reminiscent of Surival Horror.
Let's not forget A's Venom Myotismon avatar which looks creepier than ever.
Wake Up Call Boss: Veemon at Flame City due to his Digimon with extremely high attack power and Garurumon in Igloo City due to his ability to control your attacks and force you to trigger his countermeasures.