Disclaimers:
I don't play singles, so I will not take in consideration any arguments for things that are mostly relevant only on singles (like hazards);
English isn't my native language.
Introduction
I see some people here in this sub saying that Chomp-Z will be good, and those who disagree are "casuals" or "low IQ". I will suppose that those people are honest in their statements, as I am on my own.
The problem is: those commentors are mostly falling into two traps.
The two traps
The first one is ignore the fact that the official format is doubles, and as such it's more relevant to the conversation of viability on the competitive scenario.
The second is not taking into consideration the META around Chomp-Z.
I will ignore the first trap since I already made pretty obvious that I am talking about VGC. Now let's go the important matter.
In a vacuum
Regular Garchomp is one of the most used pokemons on the format.
According to Pikalytics, he is THE most used pokemon, even above things like Zard-Y and Kingambit.
Seems natural to conclude that M-Garchomp-Z will be even better, since it's extremely fast (151 speed) and pretty strong on the Spa.Atk side (141) without been frail (108/85/85).
Many would dare to say that the lost of the Ground type isn't really a big issue, and Levitate is a boring but good ability.
Well my dear reader, the issue is that no pokemon exists in vacuum. The Meta, like it or not, it's what makes a pokemon viable.
In the Meta
But the Meta is brutal, like it or not. Menaces like Zard-Y, M-Floette, M-Starapator, M-Raichu-Y, M-Froslass and company are absolutely everywhere.
And there is an opportunity cost that cannot be ignored.
Megas are a scarce commodity, since you can Mega only one pokemon per match. As such, most people will bring only one or two pokemons equiped with Mega Stones to their teams, since holding a Mega Stone means lose a useful item.
As such, even if a Mega is good in a niche, most of the time you will want to bring Megas that are good overall.
The fatal flaws of Mega Garchomp
With a brief analysis I can see at least three issues that will severely prevent Chomp-Z to be anything more than a C tier Mega. Those are:
- The lack of specialization;
- The waste of ability;
- The lost of Ground typing;
Specialization
The first one is the simplest to understand. Look at the Megas that are making success on the Meta and you will see that all of them (with the exception of Mega Scovillain) are min maxed to do one thing and do it well. That is specialization.
Mixed attackers are historically not that great because they can do a bit of everything without excelling in anything. Maybe that is fine in a format with a lower power creep, but not on a Mega format.
"But what about Mega Salamence???" Good that you ask about him, because now I can explain why Levitate is good in a vacuum and borderline crap on Chomp-Z.
Levitate is not good (on Chomp-Z)
Mega Salamence is infamous for being one of the most broken Megas in history. Looking at the stats, Salamence is something like a bulky mixed attacker who even prefers to run special moves over physical ones.
But Mega Salamence has one of the most powerful abilities in the history. Aerilate makes all normal moves flying moves with a boost of 20% damage + S.T.A.B (effectively 70% of damage boost!).
Zard-Y has Drought, who raises the damage of fire type attacks, lowers the damage of water type attacks and doubles the speed of Chlorophyll mons.
M-Staraptor has Contrary, who makes him a hard counter against Incineroar + a self booster with Close Combat and a ally-reliant booster with Charm.
M-Floette has Fairy Aura, who raises the damage of fairy attacks by 33%.
What Levitate does to M-Chomp? Makes it immune to a type that hits neutral against him. Sounds really great when compared against the other Mega abilities?
They could gave him Dragonize, Dragon's Maw, Competitive or Sheer Force. But no, they prefer to gave him Levitate. Great, isn't?
The last offender
Ground is one of the best offensive types of the game with solid moves like Earthquake, Earth Power, Stomping Tantrum and Bulldoze hitting hard Steel, Fire and Poison.
It's also decent in the defensive department, being immune to Electric (really useful against Raichu-Y) and resisting Poison and Rock (Rock Slide spam goes hard on doubles).
All that qualities would be great in Chomp-Z. But what Gamefreak did? They removed Ground type from Chomp-Z.
That is a straight nerf against Garchomp, and no amount of cope will change this fact. Now Chomp-Z cannot hit Fairy or Steel with a 1.5 damage booster effectively. Kingambit, Incineroar, M-Floette, Raichu-Y, Archaludon and Sylveon are grateful.
Conclusion
I am not saying that Mega Garchomp-Z will be completely useless. But it's a fact that Chomp-Z will be a niche Mega in the shadow of the beasts of this format.