>>16255596A lot of the megas are just not broken enough.
Mega Gardevoir has a whopping BST 618, Pixilate Hyper Voice and a SpA of 165, but only 100 speed meaning it will get outsped by a lot of things, and OHKOed because of its extremely poor physical defenses. It's also weak to Bullet Punch.
Mega Absol has a great ability and STAB Sucker Punch with 150 Attack, but has paper thin defenses on both sides and anything that can take a sucker punch has a good chance of OHKOing it.
Mega Abomasnow still has a whopping 7 weakness, and not enough bulk to make up for it.
Mega Banette is not much better than any other Prankster, and it's also one of the slowest. The only thing it has over other pranksters is priority Destiny Bond and a good attack stat (with which you can only use one or two moves if you want to fit in any good tricky moves). Prankster also does not take effect on the turn it mega evolves.
I love Mega Manetric, mostly because I am one of the only users of him, but he has a movepool that makes Flareon's look good. Not that he doesn't have any good moves, but he has maybe one or two good movesets. Funnily enough he shits all over any type of Gengar, and he also has Volt Switch+Intimidate, but he's just not going to ubers.
