User blog comment:B214/Volume 23 discussion blog/@comment-58.137.25.62-20170320022423/@comment-31137559-20170320025242

Well I would assume that having the Dragon Sacred Beast would be like having a Dragon Sacred Gear. Issei(before body was remade), Vali, Saji, and Siegfried had dragon sacred gears and they weren't dragons but their sacred gears gave them the aura of a dragon. And that made them weak against dragon slayers.