Darkrai is and must be legal in Ubers. Is cause the guy who told you probably plays on Wi-Fi a lot and has definitely no competitive knowledge whatsoever. Cause if he did, he'd know that ALL pokemon are allowed in Ubers. Heck, the only one that could possibly go beyond Ubers would be Arceus. And finally, must, cause if not, DL13 would be in a ragestorm.