A scoring system of the resulting work is fine. A scoring system that effectively encodes how they did the work is problematic because it means they company is now dictating how work be done, which is an employee relationship.
E.g. It's the difference between: a scoring system that uses a coding style standard and unit tests on the resulting code to build a score; and a scoring system that uses hours logged into the computer manipulating the IDE and lines of code written per hour to build a score (the contractor could use that as a way to score himself and for billing, but not the company).
E.g. It's the difference between: a scoring system that uses a coding style standard and unit tests on the resulting code to build a score; and a scoring system that uses hours logged into the computer manipulating the IDE and lines of code written per hour to build a score (the contractor could use that as a way to score himself and for billing, but not the company).