He's fairly effective but Mega Lucario is easily KOed by Talonflame because a speedy Talonflame with flare blitz, no hope. Which is why I don't think Mega Lucario is as OP as others say, you just have to have knowledge on how to beat it.
Actually, your Talonflame example is a false one. Most Talonflame run Adamant for the extra power due to Gale Wings' priority. However, even with 252 Spe EVs, Adamant Talonflame is in fact outsped by Jolly/Timid Mega Lucario (351 vs. 355 speed, respectively). This means your Talonflame would be forced to carry Jolly just for one specific Pokemon, which in turn lowers its damage output. And even if it does opt for more speed via Jolly, as Cloneblazer already pointed out, it's still beaten by Extreme Speed variants.
The whole point of the ban was that Mega Lucario is overcentralizing. If you look at the uber Pokemon, most of them have a small number of OU Pokemon that can check or counter them. Pokemon being banned by Smogon does not mean they have no counters; rather, it means that their counters/checks are so specific that teams are forced to carry them or risk being swept. And in Mega Lucario's case, given its wide movepool and access to Nasty Plot and Swords Dance, the things that counter or check one set do not counter or check another set, such as the Talonflame example with SD Lucario carrying Extreme Speed.
Plus, there are different mindsets between VGC/Battle Spot/regular Nintendo formats and Smogon. The default battles operate under the assumption that for every Pokemon, for every move, for every strategy, there exists another Pokemon, another move, another strategy that can counter it. And to an extent, this is true.
Smogon, on the other hand, operates under the assumption of practicality, that some strategies have counters that are so obscure, so specific, etc. that they hinder team building options by requiring you to utilize strategies that are otherwise useless outside of their specific niche countering situations. Sleep Clause exists because it's not always practical to carry a Magic Bounce user or Insomnia user in order to counter Spore spamming. Evasion clause exists because perfect accuracy moves, etc. tend to be of little use when evasion isn't used. Things like that.
Yeah, having your favorite Pokemon banned sucks, but it's done not out of spite or rage out of being beat by a Pokemon, but out of careful consideration and voting as to whether a Pokemon is too centralizing that teams have to be built specifically around that one threat.
AND STOP LOWERING MY DARN REP! I SAID TO PM ME! Yes, now I am mad. You want a reason?! I've said! I view Mega Lucario as very frail! I have said that! Oh now you're going to lower my rep for raging now?! Well it is your fault now! And STILL you don't put a name! Why don't YOU actually PM me and we'll settle this! (No I won't lower your rep on every post you make, I'm not you.)
Whoa, chill. Rep is largely arbitrary and doesn't really serve any point. Don't let it get to you.