I feel that they do slightly different things.
The barbell will give you more aggressive acute tissue manipulation.
The voodoo flossing will help all around joint tissue and also help push blood into knooks and crannies that have poor blood access as well as providing a “flush” once the compression band is released.
So for “more effective” improvements – it depends on what you’re weak on. If you have specific tissue tightness and can access it with a barbell, then the barbell is the most effective.
If you are recovering from an injury or need overall tissue improvement in, on, and around the joint, then flossing would be the most effective.
Both of those, and band distraction, lacrosse balls, etc. are all TOOLS. The tool that will be most effective depends on the job at hand.