Certain contracts are disallowed on the basis of public policy — not because they are unconscionable or illegal. Public policy limitations are extremely diverse and highly dependent on the circumstances, and on your particular jurisdiction. If you believe that your contract may involve some conflicts with public policy, make sure to assess the applicable law in your jurisdiction and consult with an attorney.
For example, some jurisdictions allow couples to enter postnuptial agreements that will affect property distribution in the event that the marriage comes to an end. Other jurisdictions believe that allowing postnuptial agreements will promote divorce, and as such, they disallow them on the basis of their public policy of promoting marriage and discouraging divorce.