In my experience, pre-existing conditions, carve-outs, and similar issues are entirely at the discretion of the carrier and are not always specifically addressed in the initial contract. I am not aware of any laws or regulations which require them to meet minimal standards.
As such, they often exclude, deny, and carve out in inconsistent ways, sometimes forever, sometime for some arbitrary waiting period. And when coverage is reinstated, there are often constraints on the coverage. It's the fox and the hen house, just another area that "needs killin'" when health care reform finally happens.
Not to mention the battles: it is very hard in some cases to know whether an event is related to an excluded condition or not. For example, migraines can rarely cause strokes or heart attacks; if migraine is excluded and the patient has one of those events, the carrier may well deny it, yet the causation is statistically unlikely. Or pre-existing epilepsy is excluded and a seizure results in an accident - will the insurer cover the broken bones or will they say the seizure was the cause so they won't cover it.
A very bad decision to leave to the insurance company.