With its triumphant blasts of accordion and relentless norteño rhythms, the wild pace of Un Chamaco Sin Futuro may leave you so breathless that you miss its lyrical bite. Legado 7 love to drive dance floors into a joyful frenzy with blazing jams like “El Nino,” and even when they slow things down on swinging gems like “Otra Vez,” they can’t hide their swagger. But despite their well-earned rep as bad boys, the quintet’s sharply observed tales of immigrant life along the border are surprisingly heartfelt, even if they can rarely resist throwing in a wicked punchline or three.