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One of it's greatest flaws is that it's still weak against fairy.
OU Pokemon with Play Rough already OHKO regular Altaria. Considering how it's obviously not getting much investment into it's defenses when it mega evolves, this isn't a good thing.
One things for certain it's going to need Earthquake or Fire Blast if you plan on using it offensively.
Scenario: Let's say M-Alt get's 120 Attack when it Mega Evolves. For it to guarantee a OHKO against M-Mawile it would need to have +2 attack(most likely through Dragon Dance).
>+1 252+ Atk Altaria Earthquake vs. 172 HP / 0 Def Mega Mawile: 280-330 (98.5 - 116.1%) -- 87.5% chance to OHKO
>+2 252+ Atk Altaria Earthquake vs. 172 HP / 0 Def Mega Mawile: 374-440 (131.6 - 154.9%) -- guaranteed OHKO
Mind you this is with high over estimate on M-Altaria's Attack. Once things for certain though, Mawile may not be able to switch ion you but you can't switch in on M-Mawile. Obviously there the exception if Mawile is already hurt, but no one calculates that way.
>Oh but wait I forgot Smogon banned Mega Mawile!~
In the end what I said before can still be applied to Azumarill.
>252+ Atk Pixilate Altaria Return vs. 228 HP / 0 Def Azumarill: 271-321 (68 - 80.6%) -- guaranteed 2HKO
>+1 252- Atk Pixilate Altaria Return vs. 228 HP / 0 Def Azumarill: 333-393 (83.6 - 98.7%) -- guaranteed 2HKO
>252+ SpA Pixilate Altaria Hyper Voice vs. 228 HP / 0 SpD Azumarill: 238-282 (59.7 - 70.8%) -- guaranteed 2HKO(I applied the same overestimate with 120 SpA)
In the end you guys are over hyping Mega Altaria way too much just like you guys were doing with Mega Mence. We can only wait till it or it's stats are released then calculate or test it ourselves to see if it can fare well.
Also
>2014 still playing by Smogon's rules
Even in VGC I doubt it will fare any better but we can only wait till it's released and test it ourselves.