Hard disagree. I've been using it on Ubuntu for over a year now and it's worked absolutely perfectly
[1]: https://github.com/ProtonVPN/linux-app/issues/110
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