this is a major issue not only related to nethserver , it is there with zentyal , clearOS , and other server
it is inability to filter out https protocol
you can check
https://www.clearos.com/clearfoundation/social/community/how-to-block-https-www-facebook-com
you can find tons of topics about this issue
lets say you want to block facebook
if you chooses transparent , then you will only block http://www.facebook.com
but at the same time you can easily open https://www,facebook.com
if u choosed transparent with ssl then that all block all the https protocol website like the online banking and shopping and every single website that uses the encrypted protocol https
this is a huge issue related to https web content filter and IMHO will never be fixed