I think there is a misconception of value here, regarding Meganium's ability.
While, on paper, it seems incredible... in practice, it will almost always be clunkier than Rare Candy.
Especially since, being a stage2 itself, you'd want to Rare Candy Meganium anyways.
If it was a good attacker, it would synergize with itself.
But it's not.
It needs to be paired with another stage2.
and once Meganium has evolved your attacker, it becomes wasted bench space.
Yes, it can consistently string attackers late-game.
But most Stage2s don't need that. You're compromising early set-up consistency, for unneeded late-game set-up.
Similar argument arose when we saw Illuminate Shiinotic from SM.
TL;DR
Rare Candy > Meganium
Even excluding Rare Candy, I'd rather set-up with Leafeon's GX, or Grovyle's ability.
Those are both more consistent, and both Pokemon lend support to a deck, outside of just evolving your Pokemon.