I believe that for a high level attacker such as ISP, Google, Facebook, etc, over time it is fairly trivial to associate a regular users web traffic, probably even easier when using a wide variety of VPN end points.
For example, if a user scrolls Instagram and streams music (or uses any of their dozen phone apps) a link could shortly be established that the two accounts consistently log in at the same time using the same IP address. A very privacy minded individual break this link by mono-tasking and changing VPN endpoint for every task.
It would be far easier to use something like Safing that creates a different channel for each app or website visited.