HIPAA addressed those issues with anyone that has employer or group supplied insurance. You can't be denied coverage, pre-existing doesn't come into play if you had "creditable" coverage before, and there are fairly strict time limits on how long they can claim pre-existing if you didn't have coverage. Since it really isn't an issue with employer provided insurance, certainly not an issue with medicare or medicaid, the people that it effects is those that purchase their insurance direct from the insurer. That's mainly because they don't have a "group" to spread the risk.

The state of Kentucky makes the insurance companies that provide policies in the state fund a high risk pool for those people that they don't want to cover under normal policies.
It isn't cheap, but it really isn't any more expensive than purchasing comparable individual coverage.

Employer provided insurance covers roughly 2/3rds of the population. The next largest group is Medicare.

As I've stated before, most large companies are really self insured and just use Insurance companies to administer the plan.

No healthcare plan has unlimited access to every treatment. Medicaid severally limits certain types of treatments and even Medicare has prodecures that they don't cover.