This is a confusing area. Here is the official Health and Human Services page about the subject, but this page is not very comprehensive.
http://www.hhs.gov/healthcare/rights/limits/
Take the case of cancer. Look at the list of essential services.
https://www.healthcare.gov/coverage/what-marketplace-plans-c...
I'm not sure where cancer is on that list, so I gather that cancer is not covered by the bare minimum essential services plans. From this, I believe it is incorrect to say ACA (aka Obamacare) requires coverage of unlimited cancer treatment.
Also, see this article:
http://khn.org/news/obamacare-lightens-load-for-cancer-patie...
It points out the key loophole in planning for unexpected catastrophic medical expenses is prescription medication, regardless of lifetime cap issues. Even if you can afford the annual copays and deductibles for the numerous office visits, the medication budget for treating cancer can easily blow out your budget planning by either not being covered at all, or only covered at a 50% level.
What makes this aspect especially hard to plan for is you currently have no way to find out in advance what these medications cost under specific plans. So if for example, you have a family predisposition to a certain type of cancer, then you have no way to find out how to financially plan for the possibility you might also get it, and have to start the medication for it.
In the US at least, I still firmly believe that prudently planning for severe medical conditions is simply not feasible, even with "catastrophic coverage", for families with less than $10M in liquid assets, and $40M in total assets.