Not universally true. The Mullvad client has lots of additional features to enhance privacy. Killswitch, split tunnelling (you might otherwise disconnect the VPN to use a certain app, so it can overall improve privacy), Shadowsocks, Lockdown mode etc

It's extremely high quality on MacOS in my experience. It's never crashed for example whereas Airvpn's crashes daily. It connects almost instantly. I don't think I've ever seen it give an error