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.
Gonna have to stop you right there, buddy.
1. Genesect nearly always runs a +Speed nature in addition to Choice Scarf, so outspeeding it is frustrating to say the least. Yes, it has that nasty 4x weakness to Fire, but it can always just U-Turn out of danger, making it even harder to pin down. And Sturdy itself is never an answer as Stealth Rock destroys it, not to mention Mold Breaker.
2. MegaLuke may have common weaknesses, but it's still so bloody fast coupled with ExtremeSpeed that it rarely matters. So what if he can't hold other items? MegaLuke doesn't need them to wreck face.
3. Just because a Pokemon is Legendary doesn't mean it needs to be banned ASAP (lookin' at you, Articuno). And this is about possibly banning Deoxys-S, not its other forms.
4. Mega Evolution itself will never be banned. Hell, several Pokemon are only viable WITH their Mega forms (Pinsir being the best example of this). Crying about all Megas getting banned is pointless.