The fight formats were made specifically to help control these problems as well as having creative ways to build decks. The Clan Fight format is the official format restricting all decks to their respective clans with some exceptions. G-Regulation is like clan fight but restricts all decks to any cards with its nation's flag (or any card that has been reprinted with its nation's flag), basically any card that has been released in the Vanguard G era. Extreme Fight is meant to be a Game Breaker and throws any restrictions out the window, most notably clan restrictions allowing for multi-clan decks.
Barcgal holds the distinction of being the first card on the official Banned/Restricted list, and even then, it is only banned for use as a starting Vanguard. It's not so much Barcgal itself that is broken, but the fact that it can be used in a unstoppable chain with Flogal and Future Knight, Llew to set up a Turn 2 Blaster Blade with five cards in the Soul, a perfect set-up for Soul Saver Dragon and its Soulblast 5. This combo was so potent that it destroyed the Japanese competitive scene at the time, at least in the Senior Division.
Nubatama would have been worth a complete ban... had the clan been given more than four cards. The clan's entire premise is based around forcing the opponent to discard if they have more cards than you do. The only problem is that the hand is the primary source of defense. If this clan had ever been completed, it would have been more difficult to deal with than Barcgal-powered Soul Saver Dragon.
As of Booster Set 13: Catastrophic Outbreak, it is now possible to have a mono-clan Nubatama deck. However, they now have support that has been slightly toned down from their original strategy, going from outright discarding to binding with the potential for card loss, and currently only have one of each type of trigger.
They do have another critical trigger, however it is only given out as a promo card and thus difficult to obtain.
At this point in time, they are a major part of a new Technical Booster with a new set a triggers and even more ways to control the hand.
Eradicator, Dragonic Descendant got slammed onto the Restricted list a mere three months after its release. This may have had something to do with almost every professional Japanese player switching to near-identical Eradicator/Dragonic Descendent decks upon release. The card itself is absurdly effective, having 11000 Power (making it a good defensive card), and a self-standing ability that gives you two Twin Drives (all but guaranteeing at least one Trigger) and forcing your opponent to guard both attacks.
It's also worth noting that his self standing ability comes into play when he's guarded, AND giving him an extra critical to boot, making him deal two damage if the attack hits instead of one (and that's if the user doesn't get any other criticals during any of their drive checks). During the late game when this skill is most likely going to be going any hit from this guy will spell endgame for the opponent.
And even with being restricted to two per deck, Eradicator decks running Descendant are STILL topping in most tournaments in Japan.
It has finally been unrestricted.
Before Dragonic Descendant, there was the infamous Dragonic Overlord The End, which was the undisputed king of the Japanese meta for a year. Its self-standing Persona Blast allowed for two Twin Drives if the first attack hit, practically forcing the opponent to guard every single attack from The End. With a power of 13k (when crossridden) it was a powerful defensive Vanguard, and was backed up by a clan that could snipe any Rearguards who posed a threat. The End was finally stopped when it was put onto the Restricted list, and only came off in a (failed) attempt to wrest control away from Dragonic Descendant.
With the release of Dragonic Overlord "The X"(pronounced "The Cross") in G Booster Set 1, a Legion Revival for The End, Dragonic Overlord once again continues his reign of control. While he has a few competitors, "The X" is arguably the undisputed king of the meta-game at the moment.
"The X"'s dominance caused Bushiroad to limit Calamity Tower Wyvern, a card used to facilitate the deck's ability to re-Legion and grab copies of The End and The X (both Persona Blasters), to a maximum of two per deck. This was not seen as an effective way to hinder the deck by English players, but Japanese players typically played the card at four copies. This is currently Japan-only.
For those that don't already know, Calamity Tower Wyvern is a unit which Soul Blasts 2 units to draw a card, note that nearly every clan has a unit like this at this point of time. You combine this with "The X's" ability to find a copy of itself or "The End" and you have an incredibly powerful loop where all you have to do is re-ride, call Calamity Tower Wyvern to remove any copies of "The X" to search out with the newly ridden "The X" and continuously take advantage of the draw, replacing triggers and overall have a huge advantage.
Bushiroad has also gone on record as stating that they plan to monitor "The X" in the future to determine if future restrictions will be necessary.
Hot on The End's heels was Majesty Lord Blaster. Like The End, it could also force the opponent to defend every turn by having 2 Critical permanently simply by sacrificing two units, both of which could be easily searched from as early as Turn 3. Add onto the deck's capacity to retire important units with one of those pieces and you had a deck that could gain advantage ridiculously quickly. Like The End, it was soon restricted after a year, but unlike The End, it has still not yet been totally unlimited, with only two copies of it allowed in a Japanese deck.
And now, he's back to being unlimited.
Lizard Soldier, Conroe, widely regarded as the best starting Vanguard that the Kagero clan has ever received. The ability to add any Grade 0 or 1 to the player's hand at any time proved to be ridiculously versatile, especially after the metagame evolved to favour Perfect Defence cards (which are Grade 1s). The effect was good enough to make every other starting Vanguard, even those dedicated to supporting specific archetypes, sub-optimal.
Conroe eventually joined Barcgal as one of the only Grade 0s banned for use as a starting Vanguard. The tipping point was the rise of Dragonic Nouvelle Vague decks that would circumvent the deck's inherent Grade imbalance by using Conroe to search for Nouvelleroman Dragon, a card that would essentially turn all of your Grade 4s into Grade 3s.
Note that Nouvelle Vague was the only legal Grade 4 card in existence at the time. The only one other was illegal due to its own effect and it can be argued that Nouvelle Vague is highly superior.
It should also be noted that Mecha Trainer does the exact same thing as Conroe except he is used with the Spike Brothers clan. The only reason why Mecha Trainer has not seen a ban or a restriction at all is simply because the Spike Brothers clan simply does not have the wide and versatile Grade 0 or 1 pool that the Kagero clan has. In other words, Conroe is banned and Mecha Trainer is not simply because the Kagero clan is better.
However, Mecha Trainer is under surveillance by Bushiroad and will probably be restricted by Conroe should it proves too dangerous.
Cat Butler has been restricted to two in Japan due to its interactions with Legion vanguards such as Mega Flare.
To be more specific, most Legion units at that point in time are a Grade 3 Vanguard with a Grade 2 attached to it. Cat Butler's effect says that when a Grade 2 Vanguard fails to hit, Cat Butler can remove itself from the field to stand the Grade 2 Vanguard again ready for another attack and, more importantly, another Drive Check. This also applies to the Grade 2 Legion Mate which, if the Grade 2 Mate stands, the Grade 3 Legion stands as well, ready for a 20k+ attack with Twin Drive ready to go. What makes this strategy effective is the amount of Cat Butlers you can use in a turn as well as the units that it can stand. In particular, Ultimate Raizer, Mega Flare can constantly threaten players with an extra critical and Phoenixraizer, Drill-Wing can stand Rear Guard units for an all out attack.
The Thing Saver "Abyss" deck. A variant of Royal Paladin decks centered around Seeker, Thing Saver Dragon that used the Shadow Paladin legion unit Phantom Blaster "Abyss" to build up soul, so that Thing Saver's skill could be used twice per turn. It consistently topped tournaments, causing Bushiroad to put a term to the deck's existence by changing the rule that allowed Royal Paladin players to have up to 10 Shadow Paladin cards in their deck to a new one that allows them to have up to four "Blaster Dark" in their decks.
Seeker, Thing Saver Dragon, even without the help of Phantom Blaster "Abyss", has been a dominating factor in the competitive scene since its release in Booster Set 16 rivaled by only a few decks. Like with "The X", Bushiroad has promised to monitor Thing Saver Dragon's future performance and, should it retain its hold on the meta-game, will receive restrictions.
Commander Laurel, a card which has been with the Dimension Police since their formal debut in Booster Set 4, is being restricted on the eve of the release of the clan's first Extra Booster, Cosmic Roar. Unlike previous restrictions, this is the first card being limited to a single copy per deck. Laurel's ability to turn any Dimension Police Vanguard into a restander is a powerful ability which has been a powerful tool and crutch for the clan, which has only grown more powerful in the era of Strides and their Triple Drive ability. Given the preemptive nature of his restriction, Bushiroad must be confident that his ability would be too much when combined with clan-specific Strides and the other planned support. This is currently Japan-only.
This was possibly in consideration based on pre-existing strategies involving the Daikaiser line of units, which can remove guard, especially Perfect Guards on top of the Criticals they gain on top of that, and Sin Buster, which stops Grade 1 or higher units to be normal called to the Guardian Circle, mainly Perfect Guards. The presence of Laurel supported what would have been a strong attack into a near guaranteed win.
Laurel has since been unrestricted but remains under surveillance.
Jewel Knight, Swordmy hasn't been restricted yet but it's on Bushiroad's watchlist and for a good reason. It has the ability to call any grade 1 or lower Jewel Knight without needing a specific vanguard by counterblasting a Jewel Knight card, leading it to be played in many Royal Paladin decks such as ones focusing on the above-mentioned Thing Saver Dragon or the Sanctuary Guard cards.
As of 12th January 2016, Swordmy cannot be used in the Clan Fight format unless all grade 3 units in the deck are Jewel Knights. Internationally, this ruling starts on February 1st of the same year.
Flash Ripple, Odysseus is restricted to two in Japan due to it's incredible field it can create and the ability to fix the Ripple ride chain. Basically, Odysseus puts one unit into soul to allow the player to superior ride a unit of the same grade from deck as well as replacing the unit that was put into soul. This would normally be designed to, as stated above, fix the ride chain so that their skills would go off effectively. However, it can create a combo with Tidal Rescue Sea Turtle Soldier whose ability to possibly call a grade 1 or 2 from deck building field very easily. And note that Odysseus can be used in any grade which supports the infamous Grade 2 rush game and can go as far as allowing the player to ride to Legion, put 4 cards (preferably triggers) back into the deck, fail to find the mate, use Odysseus to re-ride to another Legion and Legion from that putting even more triggers back into the deck and call the other Grade 3 unit out.
The final nail in his coffin was probably the finals of the world championships shortly before the restriction was announced, where the second game of the final match was decided in five turns due to a rush enabled by Odysseus.
This has also seen a restriction internationally.
As of March 17, 2016, two cards that have been restricted to 1 are the Angel Feather unit Doctoroid Refros, and the Neo Nectar unit Cosmos Pixy, Lizbeth. Both are stand triggers which returns themselves back into the deck and draws if the conditions are right. Because of how they are utilized, they both can cause an infinite loop where, if their deck is small enough, they can continuously keep using the skill over and over. This also causes slow play which the player can take advantage of.
Of the two, Doctoroid Refros is easier to understand why is has the restriction. It Counter Blasts one damage to send two cards (one of them can be the damage you flipped over basically making the skill free) and itself back into the deck and afterwards, the two damage gets replaced and the player draws one. If the deck is small enough, chances are the card they drew back is another Refros. This allows them to recycle triggers and important cards back into the deck but that's not all. Several units that gain power when a card is put into the damage zone like Nurse of Broken Heart can take advantage of this strategy acquiring near endless power forcing the player to use their Perfect Guard or take the damage.
Speaking of Nurse of Broken Heart, she is currently on the watch list due to her ability to give power to both herself and the Vanguard when something hits the damage zone, especially on defense.
Cosmos Pixy, Lizbeth is a harder nut to crack. She is a Bloom unit (in which if another copy of that unit is called, all copies of that unit can gain power and extra abilities) which allows her to put herself back into the deck along with any normal units of a single name, drawing a card after shuffling. However, the player can choose to send zero copies of that unit back into the deck, putting back the stand trigger and drawing and if the deck is small enough, chances are, it will be another one. Once again, this allows her to cause an infinite loop as long as one of the original copies of that stand trigger exists on the board. This combines well with Sacred Tree Dragon, Multivitamin Dragon who gives up to three units another 5000 power when another unit of the same name enters the board and with easy access to units that gain another's name, this also creates near endless power. To also help the deck, Maiden of Sweet Berry allows them to pull out copies of an existing card by putting itself back into the deck, thus drawing Sweet Berry is the same as drawing a Lizbeth, if not better.