In the end, we can only rely on anecdotal evidence, but my experience is that it doesn't work on NixOS and the issues are also mostly full of people who encounter multiple problems [1, 2, 3].
I don't know what I did to get so lucky. I'm on Ubuntu and have the following versions: cli@3.13.0, protonvpn-nm-lib@3.14, proton-client@0.7.1
I've even written a couple of tools that manage my connection through the CLI automatically and it all Just Works. The only issue I ever had was when I had to force shutdown my machine for an unrelated reason and I didn't have an internet connection until I opened and closed Proton VPN. I'm sure someone smarter than me could have just reset the interface they were using or something
[1]: https://github.com/ProtonVPN/linux-app/issues/110
[2]: https://github.com/ProtonVPN/linux-app/issues/109
[3]: https://github.com/ProtonVPN/linux-app/issues/96