Assuming Mozilla isn't compelled by law to share it's entire database of user information on a rolling basis without a warrant, I suspect (in the U.S.) it would be somewhat effective at shielding yourself from bulk metadata collection (government mass surveillance) of your online communications by obfuscating that metadata.
Compare this to your ISP and telecom providers. A subset of the larger providers willingly handed over the communication metadata of their users without warrant.
You know what they say about assumptions.
Because most peoples threat model doesn't include actors that can force a VPN provider to give up their data. They just use it because it's making it easier to not get data stolen in a coffee shop and watch US Netflix.