I'm also new here and tested a NDx8 filter before.
Just to contribute my personal opinion :
It will reduce the amount of light depending on the x number. That means the whole photo is affected - everything will be darker. Not get only the bright spots / reflections to be less bright and the rest of the pic will be darken also.
showtime explain it all.
For situation like taking landscape with lotsa bright sky, use a GD filter so the foreground can be expose correctly while the GD filter cut down the sky for half stop to 2 stop depending on which GD you use.