I still haven’t done this. I run into problems in almost every project because of it. I think a very long winded discussion about this basically boils down to:
Employees need skills. Businesses need successes.
Employees hand a resume to a company that has a list of skills on it; programming languages, frameworks, databases, other technical achievements. Employees are evaluated on how well they preform a task, not how well their past projects have gone. This is why candidates for a development position go through coding interviews. They don’t have strategy brainstorming sessions about how to improve their last project.
Businesses need successes. As a freelancer, you are a business. If you walk away from a project with nothing more than another notch on your resume, then you have to really ask what you gained from the project. Money disappears. Technology gets out of date really fast.
You need to walk away from a project with another win for your portfolio. A business needs to show clients that there is money to be made in this relationship. Your clients should do business with you because you helped previous clients achieve their goals.
This has nothing to do with Python, Hadoop, Node, or any other technical skill. This is all about your business improving the bottom line of your client’s business. All your previous successes prove you have whatever skills are necessary to do that.