For The Netherlands: If the car breaks down during the booking period and cannot be fixed at the location of the breakdown you will get a replacement car for 2 days or until the end of the booking period, whichever comes first. For an overview of insurance coverage in case of a breakdown, please see this article.
For Sweden: If the car breaks down during the booking period and cannot be fixed at the location of the breakdown you will get reimbursed for the cost of travel back home, or to where you were headed. Contact Viking through the booking page of the app for more details.