Well, wait until you get on Medicare. There are no maximum out-of-pocket limits with Medicare, unless you get a Medicare Advantage Plan (which is almost always an HMO).
Medical underwriting is permitted for Traditional Medicare Part B supplemental/Medigap plans, so pre-existing condition clauses apply, even with the Affordable Care Act.
In other words: once you go on a Medicare Advantage Plan (an HMO) you can never truly go back to traditional Medicare.
If you have cancer or a rare disease (it's not uncommon to have a rare disease--about 7% of the general population collectively has some sort of rare disease) you likely cannot risk being on an HMO if you want to stay alive.
I have 2 rare immune-mediated neurological diseases affecting my peripheral nervous system, and I have traditional Medicare. I require a blood product, called subcutaneous immunoglobulin (administered in that form--it is the only medication that has ever worked for me and has put me in pharmaceutical remission).
If I come back to the United States, I can expect to pay $50,000+/year for my healthcare (mostly due to the subcutaneous immunoglobulin) due to something called the Medicare Part D catastrophic coverage level.
A lot of people, and I mean a lot, get screwed due to the part D catastrophic coverage level. Actually, because of this "program" I never plan on living/working in the US ever again, unless things drastically change.