Ease of setup. That’s pretty much it.

I need no advanced features, and I have no other WG servers to connect to besides Mullvad, so I’m simply using their app which handles everything for me.

Mullvad supports split-tunnel. Sure you can somehow set that up manually with the standard clients, but with Mullvad you can simply run a command with "mullvad-exclude" and the process will be exempt from VPN. Pretty convenient.