It can deal 50 damage for a single Fighting Energy, which one-shots every non-ex version of Eevee by hitting their Fighting weakness, and Rare Candy allows it to come down before Eevee gets to evolve when going first
The most notorious offender here is Drannith Magistrate , a one-generic, one-white creature that only lets you cast spells from your hand meaning any cards with the madness ability will be discarded into exile as normal, before promptly going into your graveyard
Remember how we talked about an infinite combo with Cauldron Familiar, and Ygra, Eater of All
Japan sits slightly ahead of the global meta
Bastion Protector keeps Sigarda alive, and Riders of Gavony can make your team unblockable in many scenarios
This card also synergizes extremely well with other cards with effects that rely on cost, and there is the concern that any future cards which rely on cost will have to be developed with this card in mind, overly constraining future card development