The problem with fixed aperture drones is that you need a wide range of ND filters if you want to take your video shooting seriously. For example, on a drone with a variable aperture, you could set your exposure for F4 with a ND16 and avoid having to land and swap to a ND8 or ND32 simply by adjusting the aperture to F2.8 or F5.6 respectively. On a fixed aperture drone, if the lighting changes mid-flight, your options are:
1) Land and swap ND filters
2) Adjust a much more compromising parameter of the exposure triangle (ISO or shutter speed)
3) Deal with it later in post
Polarizers are a nightmare to use on drones (if you care about good, even footage anyway) because the level of polarization changes relative to your orientation to the sun. At 90 degrees the polarization is strongest and at 180 degrees there is no polarization. So, if you want even footage, you need to fly a carefully planned route with no deviation to your angle to the sun. That's often not practical and honestly just annoying. With a traditional camera you don't have this issue, because you can simply reach to the front of the camera and rotate the polarizer to adjust the polarization level. For still photos on a drone, polarizers are slightly less annoying, but you still need to set the orientation of the polarizer on the ground with prior knowledge of the direction you will be facing to take the picture once in the sky. After you take your photo(s) you need to land the drone and put a regular ND filter back on, again assuming you want nice even footage. One last complication is that polarizers on wide angle lenses are not uniform, so you will often times end up with parts of the sky that are darker than others.
Regarding ND filters, because the
M3P has a fixed aperture of F1.7, which is quite fast (slightly more than 1 and 1/3 stop faster than F2.8), you need stronger ND filters than you do on most other drones all else equal. I would buy ND8 up to ND64 at least, probably also ND128 if you ever shoot above light colored sand or snow in sunlight. Compared to a
Mini 2, for example, which has a fixed aperture of F2.8, you need a ND filter a full stop stronger to get a similar exposure, all else equal, so if you were using a ND8 on your
Mini 2, you would need a ND16 on the
M3P in the same conditions and exposure settings.