Personally I don't think Mega-Lucario or Genesect are broken (though TBH I hadn't used the Genesect at all nor do I have a Deoxys)
* Genesect is slow and reques an item for type changing meaning that if it runs Scarf it would be stuck with a 4x Fire weakness. On top of that some pokemon have Sturdy (granted few if any have access to both at the same time)
* Mega-Lucario still has the ironic weakness to fighting and the fact that you need the Mega stone meaning that he can't use Choice items/Leftovers/Lie Orb
* On the other hand, we have Deoxys as a Legendary with the ability to change formes to be anywhere from a Glass Cannon (Attack forme) to a Stone Wall (Defense forme) to a Jack of all Stats (Normal Forms) to a straight up Lightning Bruiser (Speed Forme) and yes I go to tvtropes and no I did not say "Fragile Speedster" because as a legendary his "bad" stats are still gonna be on par with other OU mons. I find it ironic that its the Lightning Bruiser sets that are rampaging around OU
TL;DR? If any of the three is getting the ban hammer it would be Deoxys, and if Mega Lucario is getting banned then we might as well ban Mega-Evolutions altogether.