For one thing, as I understand it, the ACA was pretty effectively sabotaged by removing the mandate, which destroys the math that makes insurance work.

Because pre-existing conditions have to be covered, you're free to wait until you have a serious (expensive) condition, then sign up for an ACA plan. And there's no mandate that spreads that risk around to healthy people, so the population is severely skewed.

I have no definitive data on this, but it seems self-evident that the system can't work well.