Legendary cards are usually split between rare and mythic rare
This might not be a deal-breaker for you, especially if you just want custom art on an existing Magic card, but it could be a deal-breaker for iterative designers
a set of four will set players back $200+
10 Thassa, Deep-Dwelling Thassa, Deep-Dwelling is a four-mana Blue 6/5 Legendary Enchantment Creature God, which is something wed struggle to say when drunk
Theyre terribly-costed if only cast once, and by the time the Squad payments become inefficient its almost certainly too much mana to be paying up front
For example, Aven Mindcensor limits how much your opponents can search their library, while Thalia, Heretic Cathar turns all their lands into tap lands