The only megaevolution I can't justify is mega Abomasnow. He's not going to live a fire move no matter how much bulk you give him. It's rather pointless. He also should have gotten a different ability upon transformation. He's the only one I can legitimately say isn't 'worth it'. But I'm sure someone will craft a team that utilizes him effectively - it's just going to be less intuitive. Like, I don't think MAbomasnow can be your lead, because he's too precious to throw away. You have to deal with the fire users before Abomasnow can come out to play.
Just going to echo the above: Mega Abomasnow is better than regular Abomasnow in every way. 252 Positive Nature Abomasnow's ATK stat hits 311, while his Mega Form's 0 Neutral Nature ATK stat only trails behind by 5 points, hitting 306, and again, that is without any investment at all! Mega Abomasnow's Special Attack Stat (assuming the base stats on the research thread are still accurate) is 25 points higher than his regular form's highest possible Special Attack Stat. So, excluding the use of items, Abomasnow's Mega Form is already able to dish out damage (roughly) equal to or greater than his original form's ATK stats if they were fully invested in
and at a positive nature, to boot! This is fantastic for Abomasnow because now he can invest more in his (vastly improved) bulk, which will let him sponge Electric, Grass, Water, and Ground-Type attacks more easily while also letting him survive hits he probably couldn't take otherwise.
I mean, I know it's a give-in that Abomasnow's Attacking stats would be much better than the original's, but I don't know how many other mega formes can boast that they out-damage (or match) the original's absolute maximum, on both sides, with absolutely
no investment whatsoever, you know? His stats are just that much higher compared to the original.
Even if the Smogon research thread happens to be wrong, and Serebii happens to be right, Abomasnow (should, realistically, I haven't done any calculations with Serebii's stats) be hitting harder from both sides of the spectrum than offensive Abomasnow sets were ever able to.