In other words, if Mega Zygarde works in other games the way it does in Legends Z-A, then it's already well-balanced lmao

It's gonna be fully reliant on the efforts of a second pokemon in doubles, and the exact right circumstances in singles to reach Complete Form. Then it'll get MAYBE one turn to shine.

Granted, that one turn could be LETHAL, but also really easy to avoid and counter.

Not to mention, you're also wasting your ITEM slot for it! a 10% Zygarde holding Focus Sash probably gets to go Complete and can use its first turn setting up Dragon Dance. A 50% with an Oran Berry gets back a few HP from the rare near-lethal hit without going over half.

A Mega-Stoned one though?

Again, if you're only Mega Evolving to use Nihil Light, that's fine, you can do that to GREAT effect on even the most suboptimal Mega Zygarde, but Core Enforcer is a special attack. It's not good for regular Zygarde. You're wasting a move slot for a BIG, incredibly predictable "maybe" scenario

Second, in order to optimize it for Mega Evolution, you basically have to build it in a way that it'd be terrible when it's NOT Mega Evolved.

Its best stats basically swap places. It goes from a medium-fast physical attacker with high defenses to a fast special attacker with moderate defenses.

First and foremost, in order to use it, you need Zygarde to be in its Complete form. This means it HAS to fall below 50% HP.

I ran a Zygarde on a ranked team in Sword/Shield. It was one of my better teams. The only way I could consistently keep it alive long enough to go Complete was to Dynamax it.