As a general rule, I only use up to an ND-8 on mostly cloudy or overcast days. ND-16 or 32 work best in full sun or very bright conditions (such as snow.) I've been using both series of PolarPro ND filters on the
P4P and have been very happy with the results. They are very lightweight with no negative effect on the gimbal and seem to be using great glass.
In regards to polarizing filters on an aerial platform, I would argue that you're not going to be able to get a polarizer perfectly set for every shot. The time it would take to achieve this would be... well, not the kind of time I'm going to spend on location. When you're flying, you have inherently created a very dynamic environment where things change the instant you yaw or pitch your gimbal to a different degree.
Maybe it would help to explain what happens to the polarized image when doing each of the 3 axes of movement.
1. Roll - this movement is the same thing as rotation the polarizing filter on the lens. It WILL affect the image when the craft is rolled.
2. Pitch - this movement will NOT change the relationship of the polarizing filter at all - but it WILL change the amount of polarized light that is in your framed shot. Remember, polarized light is strongest when it is being reflected at a glancing angle. When you go nose down, the ground is viewed from more of a straight-on angle. The filter will have less effect BUT it is not because the filter is turned the wrong way. It is because the scene contains less polarized light.
3. Yaw - does nothing that turning the polarizing would fix. Things WILL change perhaps when yawing, but it is not due to a mis-orientation of the filter. It is because the polarized light in the scene might be different.
If you did the three movements above through a 45 degree range - roll will show movement of the polarization and it will look bad. If you pitch or yaw, nobody is going to see the same kind of effect because the polarizing filter is not ROTATING along the optical axis. ROTATION along the optical axis is the one thing to avoid, especially if you are shooting video.
The other thing that you need to understand about polarizing filters is the manner that they can darken a blue sky. With a wide lens, you will see that the sky is NOT uniformly affected. The part of the sky that is 90 degrees from the sun is the part that is most affected. If the sun is overhead, then the horizon will look darker when polarizing the sky light. If you could see BELOW the horizon and look straight down through the earth, the sky beneath your feet would not be affected. And, if the sun was ON the horizon, the effect of the polarizer would be seen as a soft BAND of darkness that goes straight overhead and around the earth.
Ie., blue skies can lose their evenness if a polarizing filter is not used wisely.
Roll is the only movement that WILL change the orientation of the filter to the the world coordinates. Pitch and yaw will not. But ANY movements will change the image frame and the amount of polarized light in the frame. But messing with the filter rotation will not do anything at all when pitching or yawing.