
I’d also like to take some time to inform people that Mega Meganium got buffed pretty hard indirectly.
-
Toxic, Toxic Spikes, and Glimmora
The addition of several new Pokemon means Badly Poison and regular Poison are much more abundant. With Flower Veil, Megamium is immune to all Status conditions. Allowing itself to switch into Toxic Spikes without worry and Toxic from different Pokemon. Glimmora would beat Mega Meganium in a 1v1 fight, but its Toxic Debris ability means Mega Meganium can help against Glimmora’s teammates, and Glimmora doesn’t want to switch into Mega Meganium either.
-
Parting Shot and Rotom
Lesser know about Flower Veil is its ability to block Parting Shot, arguably the best pivoting move in the game.
ZA already gifted Pokemon Parting Shot such as Sableye and Scrafty. Now MD has gifted even more Pokemon this move with Rotom, Cofagrigus, Runerigus, and Liepard.
This makes Mega Meganium one of the best Rotom-Wash counters in the entire game, as well as solid counters to Cofag and Runer.
You block Parting Shot and Will-o-wisp/Thunder Wave, tank Hydro Pump/Thunderbolt/Hex extremely well, and your massive SpA along side utility moves like Knock Off and Dragon Tail mean you really threaten these Pokemon extremely well.
-
Mega Match-ups
Mega Dimensions has added multiple new Megas which Mega Meganium can check or counter.

X & Y
You beat pretty much every variant of both Raichu forms. You are a bulk Grass type with Earth Power. Mega Raichu Y being notable since it gets really good Spe, SpA, and gets Nasty Plot, so it’ll be meta relevant. Especially since Raichu also gets Surf and Grass Knot, so your only answers are gonna be bulky Grass types and pokemon with amazing Special bulk.

Z
While Mega Garchomp Z gets Nasty Plot and Fire moves, Mega Meganium would win in a 1v1 since it can survive unboosted Fire Blasts, and otherwise prevent Mega Garchomp Z from coming. Not only does it not want to be hit by its Fairy STAB, but Parting Shot spam will be everywhere and works nicely with Mega Garchomp Z. So if Meganium is on the field first, it will beat Mega Garchomp Z.

*
Mega Meganium’z match up against Mega Heatran largely depends on what ability Mega Heatran gets. If its Earth Eater, no chance. Anything else, and Mega Meganium can outspeed and KO Heatran with Earth Power.

Mega Meganium outspeed Mega Golurk and also resists its Ground STAB. It then KOs Mega Golurk with Giga Drain.

Likewise, Mega Meganium can outspeed and OHKO Mega Crabominable with its Dazzling Gleam. Though Mega Meganium can't easily switch into its Ice STAB.

Zeraora lacks any good coverage moves to hit Mega Meganium hard. Its best option is Iron Tail, which is inaccurate. Mega Meganium also resists Zeraora’s STAB and many of its coverage options like Close Combat and Grass Knot.

Mega Meganium resists Tatsugiri’s Water STAB, which now it gets Scald which also means Flower Veil helps prevent burns, and is immune to Tatsugiri’s Dragon STAB. You should watch out for Ice Beams.
-
Non-Mega Match-ups

Mega Meganium resists both of Keldeo's STAB and Earth Power. Only thing Mega Meganium would have to worry about is Ice Beam, which if Keldeo is running Choice Specs can be avoided.

Like how it beats Mega Zeraora, Mega Meganium beats regular Zeraora, both forms will have niches due to their speed tiers.

Non-Mega Heatran can't do much against Mega Meganiam despite its type advantage. And unlike its Mega, which can have Earth Eater, Heatran has no luxuries and is also still slower than Mega Meganium.

Darkrai has access to Sludge Bomb and Ice Beam, however besides those 2 moves, Mega Meganium resists its main STAB and Fighting coverage. So Choice Specs/Scarf Darkrai can be checked by Mega Meganium like Keldeo.