Uninstalling Exchange Server 2013 will fail, if the PowerShell MachinePolicy or UserPolicy is set by GPO.
You will receive an error message referencing Microsoft KB article 981474, which refers primarily to Exchange Server 2010.
The following PowerShell command removes the GPO setting.
Set-ItemProperty -Path HKLM:\SOFTWARE\Policies\Microsoft\Windows\PowerShell -Name ExecutionPolicy -Value ""
After setting the ExecutionPolicy attribute to an empty string, Exchange Server 2013 can be uninstalled successfully.