I have made a basic debian jessie virtual machine with a VPN installed and connected it to the whonix gateway. Besides this I have also customized firefox with many privacy settings and extensions.
This works perfectly fine, also with VPN firewall on the debian vm. Just need to flush and reload it once. And I have also followed the instructions on this page to make a customized whonix workstation for a VPN.
But since you deactivate a few important whonix features in workstation to allow a
tor -> vpn -> internet setup, what reasons will there be left to use a whonix workstation with a VPN instead of a default debian jessie vm with a VPN?
What would be the main differences and/or downsides in using a default debian vm vs a customized whonix workstation with a VPN installed?