Oh, I am sure that it is not safe, thanks to the PATRIOT Act. Even if they were not storing any metadata, VPN providers can be compelled to 1) share all data about their subscribers, which will include you, then 2) silently wiretap and decrypt everything. US courts will rubber-stamp, as they've consistently done in the past, and "that's all, folks".

Sadly it's not like you'll be much safer elsewhere: as soon as you step outside of the US, one of the strongest cybersec agencies on the planet (NSA) will have free reign on your traffic. But you can resist the legal attack (in some countries) and at least try to make it challenging on a technical level.

I hope Mozilla want to bring some innovation to the table that will make VPNs somehow more resistant to legal attack (not just in the US) but I doubt it.

From write-ups of the 2013 leaks, we saw references to violations of the legal theory used to justify the Presidential Surveillance Program. One of those violations was them unintentionally collecting the wrong data, due to how the ISP was bundling packets or something like that, which constituted a warrantless search, and they supposedly took that very seriously because it jeopardized the whole program.

My take on the surveillance program is that they try very hard to be law abiding, even if they have to stretch what the law means to justify the program. If you are worried they have a warrant for your communication, a VPN isn't going to help you. If they don't have a warrant, they will avoid U.S. citizen's content like the plague for fear of compromising the whole program.