As the guy working on NEXT, I wanted to clarify the differences here.
There are a lot of mods like DuskMod, mods that change up base stats, movepools, and moves to put pokemon on a more even playing field.
That's not what NEXT is. The trick is in its name: "NEXT" means "next-generation", something that Game Freak could plausibly do. NEXT restricts itself to plausible changes Game Freak could make:
Base stats can't change. The type chart can't change. Ability distribution can't change. Typing can only change in very specific circumstances. Movepools can only be added to, not removed from. And core game mechanics like freezing and critical hits can be modified, but never removed entirely.
Because of that, NEXT's balancing efforts are very similar to Game Freak balancing efforts: it's focused on adding new things, and in some cases, changing existing moves/abilities/items.
A good example of how NEXT balancing works is Cherrim: we thought, "Game Freak had an idea for a dedicated Sun support Pokemon. How do we make that OU-viable?"
Pokemon Showdown | Replay player | No, I am not impressed that you know that my name is Guangcong Luo. The PS website says "Copyright Guangcong Luo" at the bottom, ffs.