I can get an MRI in a few hours for a couple hundred bucks out of pocket. I can go get blood drawn in a half hour and lab results tomorrow morning. Tons of private labs and imaging centers are looking for anyone to fill the machine or get stuck. That's not the problem. The problem is getting an M.D. to read the results and actually tell me what's wrong and a good course of action.
In both examples, just getting an MRI would have told me practically nothing. Maybe for the nystagmus, it would have told me if there was significant brain cancer. Maybe a blood test would have told me something about cancer, but there's a good chance it would have been inconclusive, I needed a biopsy (or, in hindsight, a bit of rubbing alcohol).
I've seen a case of both providers of ACA "marketplace" plans in a state each including only one of the four closest hospitals to me as "in network", with similar extremely-spotty coverage for everything else. And one excluded the massive children's hospital in the city that'd also gobbled up every pediatric care office in 50 miles, so there was, practically, only one insurer you could pick if you had kids (those two providers were the only ones offering individual insurance in that state at all, everyone else had pulled back to only doing group plans in that state; both were also companies I'd never heard of before).
This was in an actual city. Things can be even worse out in the sticks, where hospitals are much farther apart and often offer only some of the services you expect from a big-city hospital.
Lots of these plans also only cover a small geographical area, except for ER visits (which I think they have to cover). So don't get sick in a way that gets you discharged from the ER into a regular hospital bed, but still unable to get home, while traveling within your own country, if you don't want to go bankrupt.
Like it truly wouldn't have been crazy for someone on one of those plans to get some kind of travel insurance while traveling in the US. That's how fucked up our healthcare system is.