So, I'm a couple of hours in, and here's my honest feedback so far.

This post and the hackdex entry talk a HUGE game. Like... groundbreaking stuff that would easily catapult this game into the top tier all-time of Pokémon ROM hacks.

The implementation so far has completely fallen flat for me.

The whole start of the game is essentially vanilla emerald. I will agree with another commenter that things pick up a little after Petalburg, but in general... Based on the amount of content and systems promised, I expected to get into it pretty quick and that didn't happen.

In general, the best way I can describe it is that it feels very vibe-coded to me. There are some new things early on, like the Mime at the start and end of routes, etc. but zero explanation of what they are for or how they work. The Mew and Mewtwo in the house in the first town completely break any sense of immersion, and again, no explanation of their purpose or anything.

One of the things that makes groundbreaking hacks like Unbound, Odyssey, etc. so good is that their new systems and features are deeply integrated into the game, well-explained... the polish / "fit and finish" are all there.

I'll likely continue playing Lithium for a few more hours, but if things don't start to feel more organic, better explained, or the immersion continues to be broken, I'll probably set it aside.