Creatures, mostly, but Enchantments, Artifacts, Tokens, Lands, and Planeswalkers can all be Legendary
Yes, it's a hard counter for sure, but it's madness is an abysmally high four mana
But Kyodai isnt just about defenseits activated ability pumps it into an 8/8 threat, making it a late-game finisher that also protects your key pieces early on
Its fantastic in any format with fetch lands or just cheap, efficient spells to churn through your deck and fill up your graveyard
This allows for some more unique and needed reprints to be slipped into these sets
You give up damage for a turn, but in return you get a clean window to pick apart negation-stacked fields and set up a game-ending push the kind of impact you expect from the strongest Yu-Gi-Oh