I do not buy the whole ergonomic portion for most split keyboards. It feels like a justification after the fact.

That said I used to use a lily58 and for me it was great. I have a lot of papers, notes and books on my desk. A small easily movable keyboard meant that i could have something between the keyboard halves, writing and reading without issue

I have RSI issues with my wrists. It really helps to have split ortholinear keyboards, if only because I have trained on them specifically to type without moving my wrists whatsoever (using a miryoku layout), while old habits persist on a standard QWERTY TKL. Of course, it also helps to put on really soft-springed keys (like kailh silver) — it's self evident that softer keys are easier to press.

I had initially thought that it'd be hard to use both kinds of keyboards at once, but my muscle memory for either seems unaffected when I use the other one.

Conversely, I'm totally sold on it. The shoulder-hunching thing is so real.

The thing I thought was ergonomic BS was the benefits of columnar/ortho layouts; everyone talked about how your fingers just moved vertically and it was so much better for them, and I rolled my eyes. But dang if it hasn't proved to be meaningfully true for me, too -- when I have to type on a legacy keyboard, I can clearly feel the pain in my fingers. (The disclaimer here is that my fingers are totally screwed up; if you don't feel pain normally, this probably matters less.)