From my experience
Mega Absol - While it is good at at least doing damage it is still frail and pretty much gets OHKO every time.
Mega Mawile - I don't see why people are making it look like it is amazing because with such a low speed and fire type being so common it gets OHKO a lot as well. You will be relying on Sucker Punch a lot and hope for the best. Still it is better than Mega Absol unless you somehow manage to do a Sword Dance with Mega Absol.
Mega Charizard X - Its boosted stats go to the wrong place. Charizard is a special attacker but instead it gets stats on Attack. It does benifit with a good typing though Dragon/Fire. Even then it isn't fast enough and can be taken out relatively easy.
Those are the only ones ive used and Im about tired of going against Blaziken in every match. Thing is just stupidly OP. Mega Absol may actually be better than Im giving him credit for but I have to deal with Blaziken every single time
