It's not obvious that custom properties cannot be made in any item in qbs (as possible in Qt Quick qml).

Please add an error message from qbs whenever a custom property is defined that cannot be used.

Example:

Export {
    property string myProperty: "useless"
    ...
}

could trigger some error message as the property cannot be used anywhere. There are error messages when trying to use the property, but it's not that obvious that it just can't be defined there.