Gamefreak clearly demonstrated that it has no idea how to balance mons in reducing Mega Zygarde-Complete's BST boost to 70 but giving it a spammable 200 Base Power move. If Gamefreak had instead given Mega Zygarde-Complete a full 100 BST boost like every other Mega but reduced Nihil Light's Base Power to 140, like Z-Core Enforcer in Gen 7, it'd be a lot more balanced.
Just to give you an example of Mega Zygarde-Complete's power:
252 SpA Choice Specs Hadron Engine Miraidon Electro Drift vs. 252 HP / 0 SpD Mew in Electric Terrain: 436-514 (107.9 - 127.2%) -- guaranteed OHKO
252+ SpA Choice Specs Protosynthesis Raging Bolt Draco Meteor vs. 252 HP / 0 SpD Mew: 474-558 (117.3 - 138.1%) -- guaranteed OHKO
252 SpA Zygarde-Complete-Mega Nihil Light (200 Base Power) vs. 252 HP / 0 SpD Mew: 484-570 (119.8 - 141%) -- guaranteed OHKO
252 Atk Choice Band Rayquaza-Mega V-create vs. 252 HP / 0 Def Mew in Sun: 563-663 (139.3 - 164.1%) -- guaranteed OHKO
252 SpA Choice Specs Kyogre Water Spout (150 BP) vs. 252 HP / 0 SpD Mew in Rain: 612-721 (151.4 - 178.4%) -- guaranteed OHKO
252 SpA Choice Specs Solar Power Charizard Overheat vs. 252 HP / 0 SpD Mew in Sun: 631-744 (156.1 - 184.1%) -- guaranteed OHKO
252 SpA Choice Specs Beads of Ruin Chi-Yu Overheat vs. 252 HP / 0 SpD Mew in Sun: 654-771 (161.8 - 190.8%) -- guaranteed OHKO
Basically, only the strongest of weather-boosted Choiced attacks outdamage Mega Zygarde-Complete's Nihil Light, and Mega Zygarde-Complete isn't choice-locked and can boost its speed with Dragon Dance. I believe it will be an easy boot to Anything Goes, and Mega Zygarde-Complete should be centralizing even in VGC due to how powerful Nihil Light is since the move is just outright broken and is the most broken offensive move move of all time.