There are a lot of limits on this coverage. If the driver isn't on the rental agreement, no coverage. If you're intoxicated, no coverage. If you're driving on an unpaved road, no coverage. So yes, there are restrictions, refer to your cardholder's agreement for the complete details.

Also note that since it only covers the value of the car, you're not really covered against the big liability risk: Injuring another person.

That said, if you are driving in the US, and you have your own auto insurance, it almost certainly covers you when you are driving a rental car.