His main special options are Aura Sphere, Dragon Pulse, Flash Cannon, Psychic, Shadow Ball, and Vaccum Wave.
He does have access to Nasty Plot, but the common Swords Dance use of Lucario even proves that setting up with his frail self is a pain.
Generally speaking, it's not that he's not viable as a whole. He can be viable if he's in the right scenario. He's just out classed by tons of other things running around in OU.
As a physical sweeper he gets Swords Dance w/ Bullet Punch and E-Speed for priority. Or just a STAB close combat for wall breaking.
The other main thing contributing to that claim is that a couple of the moves that were useable lost some viability. Aura Sphere got nerfed by 10 power (90->80) in Gen VI, as well as Dragon pulse getting moved down to 85 base power. Dragon typing is now not so hot offensively with the massive amount of fairies running around, and the steel types running around to stop all of the fairies.
So overall, his move pool is a little shallow and less powerful than before, but the main reason they claim he isn't viable is because so many other common OU threats out class him at special sweeping.