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After 27 years of disrespect, the Chikorita/Bayleaf/Meganium line finally gain a good Ability
(hexbear.net)
Growth will give (+2 instead of +1) for both Atk and SpA, Synthesis will heal 2/3rd total health, Solar Beam will need no charge turn and Weather Ball will permanently have Fire coverage
Mega Meganium still seems pretty mid. Grass/Fairy is an awkward type combo and while 1-turn Solar Beam off of 143 SAtk is good, it's nothing special for Megas. The real shit would've been Triage as its ability.
Grass/Fairy is a decent type, Tapu Bulu showed this. It is indeed awkward, but as Tyranitar shows, it's about having enough resistances to enter the field. Grass/Fairy has all the relevant resistances to Water, Fighting, Dark, Ground, Electric. Also Mega Stone cannot be Knock Off'd. Then it can boost to +2 on switch in and fire away hard hitting attacks. Fairy/Fire/Grass is a really good coverage, Mega Meganium can even drop the Grass attack for Synthesis to heal 2/3rd HP or Knock Off for mixed attack and utility. Any Fire pokemon coming in will hate getting Knock'd Off because they are weak to Stealth Rocks.
I'd say it'll be at least UU viable.
Tapu Bulu created Grassy Terrain on switch-in, could hold an item and didn't require your Mega slot.
Even at +2 Atk, your Knock Off won't do very much damage to neutral targets. Your Fairy STAB is Dazzling Gleam which isn't very powerful either. Meganium will be very reliant on the high base power on Weather Ball and Solar Beam. It has good resistances but also a multitude of common weaknesses and while it's not slow, it's not fast either. It will be able to get a boost off fairly consistently due to its bulk, but I just don't see it sweeping teams.
I think UU viability in Gen 9 is optimistic. Imo it's gonna be ranked RU. The power level has increased a lot.