While they let in more light you just cannot film outside without ND filters, all pro-level video cameras have them built in. So ND is a must for outside, always has been.
The reason they are really needed on a DLSR is that most people are trying to get shallow DOF so that is taking away the main way we limit light. With video you are locked into only a couple of useful shutter speeds unless you want “Gladiator” motion. So if we cannot change the shutter speed and we don’t want to give up the wide open aperture for DoF we have to cut the light coming in.
Using single ND filters would be better than Faders but Faders offer a quickness and in the end are a bit cheaper than buying a set. I use a Fader and love it. I put a rubber hood on it and use it like I do the iris control, with a figure I can open up or close down, to adjust exposure, even within a shot.
Now if you want a ND and Pol, I would recommend get a fixed ND and a Pol. If you have a Fader and a Pol, even if it didn’t cause you some funky patterns it would be a bear to adjust.
Just my 2 cents.